Fix packetizer test import

Don't use pion/rtp@v2

Fix Unmarshal to used struct

Extensions field was not cleared on Unmarshal and data was appended
to the old extension slice.
This fixes Extensions field to have correct number of data even if
the slice has some data before calling Unmarshal.

Reuse Extensions and CSRC fields.

Update CI configs to v0.4.7

Update lint scripts and CI configs.

Fix randoms

- Use seeded mathematical random where uniqueness is needed
  but not required to be cryptographic
- Use crypto random where used as a password or credential

Fix panic unmarshalling STAP-A

The H264 Unmarshal method could panic
while unmarshalling a STAP-A unit smaller
than the unit's declared size

Fix AudioLevel Extension

* ID/len handled by rtp package now

Fix AbsSendTime marshal/unmarshal in Packetizer

ID/len byte of AbsSendTime was duplicated
and ID of AbsSendTime extension was always unmarshalled as 0.
(regression from edd47ed)

RTP Packet del helper + fix

* DelExtension helper funcion
* Return nil in GetExtension if h.Extension is false
* Fix extension size check in SetExtension

Fix rtp header extensions marshaling/unmarshaling

* The length in the RTP Header extension is the number of 32 bit words
  in the extensions. I.E. with rfc 8285 multiple extensions is the size
  of the all the extensions divided by 4.
* The extensions must be be padded to 32-bit boundaries.

* Fix the tests to have a correct raw packet.
* Add some more tests.

Extension Array

Store extension data as array