add additional environment variables to custom commands (#1414)

[aler9]

Fixes #1414

new variables: MTX_CONN_TYPE, MTX_CONN_ID, MTX_SOURCE_TYPE, MTX_SOURCE_ID, MTX_READER_TYPE, MTX_READ_ID

[codecov]

Codecov Report

Merging #2356 (f60cbe3) into main (ed77560) will increase coverage by 0.30%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main    #2356      +/-   ##
==========================================
+ Coverage   60.60%   60.90%   +0.30%     
==========================================
  Files         127      127              
  Lines       14448    14517      +69     
==========================================
+ Hits         8756     8842      +86     
+ Misses       5036     5020      -16     
+ Partials      656      655       -1     

Files Changed	Coverage Δ
internal/core/rpicamera_source.go	0.00% <0.00%> (ø)
internal/core/source.go	100.00% <ø> (ø)
internal/core/source_redirect.go	0.00% <0.00%> (ø)
internal/core/srt_source.go	34.56% <0.00%> (ø)
internal/core/udp_source.go	38.19% <0.00%> (ø)
internal/core/webrtc_source.go	50.44% <0.00%> (ø)
internal/core/srt_conn.go	48.27% <94.44%> (+1.70%)	⬆️
internal/core/conn.go	94.00% <100.00%> (+0.66%)	⬆️
internal/core/hls_muxer.go	56.20% <100.00%> (ø)
internal/core/hls_source.go	54.05% <100.00%> (ø)
... and 9 more

... and 1 file with indirect coverage changes

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