[FEATURE] Support a "metadata" kvssink param

[richardgavel-ordinaryexperts]

Is your feature request related to a problem? Please describe.
Unable to add fragment metadata when using the gst-launch-1.0 command line.

Describe the solution you'd like
A kvssink param similar to the "stream_tags" param, but instead of being used for stream creation, it's a set of name/value pairs to use to invoke the putFragmentMetadata method after starting the stream.

[hassanctech]

#1015

Considering this ^.

Also, the PutFragmentMetadata API allows for a new/different tag per fragment. I'm considering in addition to this a custom event which client GStreamer applications can put on the event bus which kvssink will respond to and invoke the PutFragmentMetadata API.

However as @richardgavel-ordinaryexperts has suggested, such an approach will not easily work for clients using gst-launch so we will likely need both changes, I can work on the latter.