Signed-off-by: Bogdan Drutu <[email protected]>

**Link to tracking Issue:** 
This PR partially addresses the following issues:
- Resolves: #2265 
- Related:  #2268, #2282.

**Description:**

The main idea here is to convert `stanzareceiver` into a helper package for building various other stanza-based receivers. Each of these other receivers will only vary by input operator. Functionality pulled out of `stanzareceiver` was moved into a new `filelogreceiver`. `stanzareceiver` should most likely be renamed and/or moved, but is left in its previous package for this initial PR. 

`stanzareceiver` defines an interface called `LogReceiverType` which each stanza-based receiver must implement and pass to `stanzareceiver.NewFactory(LogReceiverType) component.ReceiverFactory`. 

With this interface, each stanza-based receiver should only need a small amount of work to have a fully functional receiver. Support for parsing operations, emission from stanza's internal pipeline, and conversion to pdata format are all handled in the helper package so that these will be standardized across all the full set of stanza-based receivers.

**Next Steps**
Input operators are _not yet_ isolated to the top level of the configuration. The end goal is: 
```
filelog:
 include: [ receiver/stanzareceiver/testdata/simple.log ]
 start_at: beginning
 operators:
   - type: regex_parser
       regex: '^(?P<time>\d{4}-\d{2}-\d{2}) (?P<sev>[A-Z]*) (?P<msg>.*)$'
       timestamp:
         parse_from: time
         layout: '%Y-%m-%d'
       severity:
         parse_from: sev
```

but the current state is still:
```
filelog:
 operators:
   - type: file_input
      include: [ receiver/stanzareceiver/testdata/simple.log ]
      start_at: beginning
   - type: regex_parser
       regex: '^(?P<time>\d{4}-\d{2}-\d{2}) (?P<sev>[A-Z]*) (?P<msg>.*)$'
       timestamp:
         parse_from: time
         layout: '%Y-%m-%d'
       severity:
         parse_from: sev
```

The primary requirement #2265 is to promote the input operator to the top level of the receiver config. This will be the focus of the next PR. This PR is mostly concerned with splitting up the package. The configuration changes might be a little messy so I wanted to address those separately.

On the subject of configuration - the interface defined by `stanzareceiver` has a method `Decode(configmodels.Receiver) (pipeline.Config, error)` which is in my opinion much too loosely defined. Too much responsibility is delegated to each stanza-based receiver. The main reason this is left this way for now is that `stanza` operators do not currently use `mapstructure` for config unmarshaling. There is currently a workaround in place, but once stanza operators are migrated to `mapstructure`, more responsibility for unmarshaling should be extracted back into the helper package, and this interface method should end up a lot cleaner. I'm planning to look into this in the next PR.

**Open questions** (which can be addressed in this PR or the next):
- Should the helper package be completely standalone, or does it belong in `receivercreator` or similar?
- If the helper package should be standalone, what should it be called? (probably not `stanzareceiver`)

**Temporarily removed functionality**
This functionality will be implemented in the near future. There is some design to do on how exactly this should work when used by multiple receivers:
- Offsets database (tracked by #2287)
- Plugins (tracked as item on #2264)

**Testing:** 
Unit tests are roughly the same as before. A few cases were dropped because they no longer applied. Certainly more tests will be added as this pattern is solidified. 

Testbed scenario is unchanged and still passing:
```
> make run-tests
./runtests.sh
=== RUN   TestLog10kDPS
=== RUN   TestLog10kDPS/OTLP
... (abbreviated)
=== RUN   TestLog10kDPS/Stanza
... (abbreviated)
--- PASS: TestLog10kDPS (30.73s)
    --- PASS: TestLog10kDPS/OTLP (15.32s)
    --- PASS: TestLog10kDPS/Stanza (15.41s)
PASS
ok      github.com/open-telemetry/opentelemetry-collector-contrib/testbed/tests_unstable_exe    31.406s
# Test PerformanceResults
Started: Mon, 08 Feb 2021 13:35:08 -0500

Test                                    |Result|Duration|CPU Avg%|CPU Max%|RAM Avg MiB|RAM Max MiB|Sent Items|Received Items|
----------------------------------------|------|-------:|-------:|-------:|----------:|----------:|---------:|-------------:|
Log10kDPS/OTLP                          |PASS  |     15s|    19.9|    20.6|         39|         47|    149900|        149900|
Log10kDPS/Stanza                        |PASS  |     15s|    28.4|    29.3|         40|         48|    150000|        150000|

Total duration: 31s
```