shared/bpm: Add BPM (Broadcast Performance Metrics)

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Introduce support for delivering BPM (Broadcast
Performance Metrics) over SEI (for AVC/H.264 and
HEVC/H.265) and OBU (for AV1) unregistered messages.
Metrics being sent are the session frame counters,
per-rendition frame counters, and RFC3339-based
timestamping information to support end-to-end
latency measurement.

SEI/OBU messages are generated and sent with each IDR
frame, and the frame counters are diff-based, meaning
the counts reflect the diff between IDRs, not the running
totals.

BPM documentation is available at [1].

BPM relies on the recently introduced encoder packet timing
support and the packet callback mechanism.

BPM injection is enabled for an output by registering
the `bpm_inject()` callback via `obs_output_add_packet_callback()`
function. The callback must be unregistered using
`obs_output_remove_packet_callback()` and `bpm_destroy()`
must be used by the caller to release the BPM structures.

It is important to measure the number of frames successfully
encoded by the obs_encoder_t instances, particularly for
renditions where the encoded frame rate differs from the
canvas frame rate. The encoded_frames counter and
`obs_encoder_get_encoded_frames()` API is introduced
to measure and report this in the encoded rendition
metrics message.

[1] https://d50yg09cghihd.cloudfront.net/other/20240718-MultitrackVideoIntegrationGuide.pdf

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