Add option to cache/reuse (Widevine) license server certificates

[ftyp]

Currently the (Widevine) license server certificate is requested over and over again on attachSource() when reusing the player to change contents, e.g. zapping through TV channels.

Usually only a single or very few license servers will be used and the certificates will likely not change for a very long period of time.

It would be useful to have an option to cache and reuse license server certificates per domain to avoid additional redundant requests to the license server and improving startup time.

Cached certificates should be removed and newly requested once a DRM error is encountered.

Thanks for considering!

[dsilhavy]

Related to #4803 and adding support for the new certurl element.