add audio_webfile_start/audio_webfile_data_received/audio_stopped callbacks

[freddy36]

Adds the following callbacks:

void audio_stopped(const uint32_t pos) // called after audio has stopped
void audio_webfile_data_received(const uint8_t *writePtr, const int16_t bytesWritten) // called after data has been received from a web file stream
void audio_webfile_start(const bool isTTS, const char *lastHost) // the webfile stream starts

And adds the existing audio_eof_stream callback to the readme

[schreibfaul1]

What can you use it for?

[freddy36]

I'm using it to stream webfiles and save them to SD while playing.

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[freddy36]

I've rebased the PR. Do you have any alternative suggestion for the use case?

[cziter15]

Might be useful to cache TTS. For example I use TTS to get notified about Home Assistant TTS events.

Imagine you have TTS notifications like "Hey, a movement has been detected in basement". Instead of asking for TTS sentence, it can be cached using md5 of the sentence adding the language as salt.

Seems like a good feature candidate anyway.