Deliver player metadata

[waliid]

Description

This PR allows us to deliver the metadata of the current item being played.

Important

• The documentation hasn't been updated yet and will be done in another PR. An issue has been created for this purpose.
• During our development we've noticed that several improvements could be done. These improvements will be made in other PRs.

Changes made

• Adds delayIfNeeded.
• Removes AssetMetadata, Assetable.
• Introduces MetadataAdapter, AssetContent, CommonMetadata, TrackerLifeCycle.
• Renames PlayerMetadata to PlayerItemMetadata.
• Introduces published property metadata on the player.
• Fixes receiveOnMainThread.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).