[Opus] Rewrite timestamps

[mat-hek]

Currently, elements in the https://github.com/membraneframework/membrane_opus_plugin don't rewrite timestamps:

• The parser generates timestamps based on frame duration
• The encoder and decoder ignore timestamps

The desired behaviour would be:

• The decoder should set the output PTS of each output buffer to the PTS of the corresponding input buffer
• The parser should have a generate_best_effort_timestamps option and generate PTS only if it's set to true
• The encoder and parser (if generate_best_effort_timestamps is set to false) should set the output PTS in the following way:
  • If an input buffer generates multiple output buffers,
    • the PTS of the first output buffer should be equal to the PTS of the input buffer
    • the PTS of each subsequent output buffer should be increased by the frame duration
  • If an output buffer is created out of multiple input buffers, the first of them should be treated as the origin of the output buffer

This is a part of #632