Public API for manual replay control

[romtsn]

Similarly to web replays, we should provide public API for controlling replay behavior manually:

• start/startBuffering should start recording a replay regardless of sample rates set, with the corresponding mode
• stop should stop the replay and send it to sentry
• flush should flush all pending segments/frames to sentry

[cmanallen]

@romtsn Is this done?

[romtsn]

@cmanallen only on iOS I think, no other platform

[billyvg]

We should probably add these to the other platforms before GA so that our APIs are consistent.