chore: Add MANIFEST_LOADING_STARTED event

[vodlogic]

We have a requirement to refresh the CDN token on the manifest URL. We found that updating the URL using attachSource() added a black screen for about a second, so we are now using this event in conjunction with RequestModifier.

[dsilhavy]

You are using this event in your application? Then please add it to MediaPlayerEvents , CoreEvents are internal events that should not be needed at the player level.

[bbert]

Hello @vodlogic @dsilhavy
Another way to resolve this issue of MPD url token expiration is to propose an API to update the manifest URL for the session.
For example: MediaPlayer::updateManifestUrl(newUrl)

Thus, once the token has expired, you would be able to provide (by anticipation or once you get an (403) error on manifest update) the new manifest url without having to implement a RequestModifier.

What do you think?

[vodlogic]

I personally think that having the new API suggested by @bbert would be beneficial as it would reduce the overhead of parsing the existing URL in the event. I'm still getting familiar with the innards of the player, so would welcome your input on what variable / property the updateManifestUrl() function should update.

The event in this PR may still be useful in some circumstances once its been moved to an exposed MediaPlayer event