Make transport mode optional

[toreonify]

When library establishes connection to RTSP server and send a SETUP request, it always sends mode parameter. For some reason with this parameter set server returns error 461.

FFMPEG (ffplay) doesn't send mode parameter and establishes connection successfully. When I removed
mode setter from pkg/headers/transport.go b/pkg/headers/transport.go in base.HeaderValue it worked fine.

As I looked further, mode can be nil but it's always set in client.go in function doSetup.

Maybe add an option to disable this parameter or retry without it on error?

Same commented out block is also mentioned in #574 (comment)

Version info

I'm using older version, but client.go still sets mode in newer versions too.

Go: 1.21.11 linux/amd64
gortsplib: v4.8.1
MediaMTX: bluenviron/mediamtx@6513f05

Information about RTSP server from DESCRIBE:

RTSP/1.0 200 OK
CSeq: 2
Content-Base: rtsp://<connection url>
Content-Type: application/sdp
Content-Length: 264

v=0
o=- 0 0 IN IP4 127.0.0.1
s=No Name
t=0 0
a=tool:libavformat 58.29.100
m=video 0 RTP/AVP 96
a=rtpmap:96 H264/90000
a=fmtp:96 packetization-mode=1; sprop-parameter-sets=Z00AMp2oCoAv6EAAAPoAADDUIQ==,aOqPIA==; profile-level-id=4D0032
a=control:streamid=0