add statistics to Client, ServerSession, ServerConn, ServerStream (#556)

[aler9]

Fixes #556

TODO:

• per-media statistics
• jitter buffer, localSSRC, remoteSSRC, last rtp packet sent RTP time, last rtp packet received RTP time

[codecov]

Codecov Report

Attention: Patch coverage is 79.60199% with 164 lines in your changes missing coverage. Please review.

Project coverage is 82.01%. Comparing base (8c4a3ca) to head (27dbba1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
server_session.go	70.23%	38 Missing and 12 partials ⚠️
internal/rtcpreceiver/rtcpreceiver.go	14.28%	22 Missing and 2 partials ⚠️
client.go	89.41%	12 Missing and 8 partials ⚠️
server_session_media.go	86.40%	10 Missing and 4 partials ⚠️
internal/rtcpsender/rtcpsender.go	25.00%	11 Missing and 1 partial ⚠️
pkg/rtcpsender/rtcpsender.go	9.09%	10 Missing ⚠️
client_media.go	90.32%	7 Missing and 2 partials ⚠️
client_format.go	85.71%	4 Missing and 2 partials ⚠️
server_session_format.go	88.00%	4 Missing and 2 partials ⚠️
pkg/rtcpreceiver/rtcpreceiver.go	28.57%	5 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #656      +/-   ##
==========================================
- Coverage   82.36%   82.01%   -0.35%     
==========================================
  Files         120      120              
  Lines       11888    12413     +525     
==========================================
+ Hits         9792    10181     +389     
- Misses       1709     1818     +109     
- Partials      387      414      +27     

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