Embed Jitsi as a PlatformView

[kamskr]

Hello!

This PR attempts to implement changes discussed in #36.
I took the changes proposed by @geiszla in https://github.com/BindrUK/jitsi_meet/tree/platform-view

In addition to iOS NativeView implementation this changes also contain:

• added a feature flag proposed in Add feature flag to enable/disable the prejoin page. #67 also mentioned in add prejoinpage constants #70 as it was needed in my project,
• fromJson and toJson methods to JitsiMeetingOptions for ease of passing them through the methodChannels

I'm still working on the Android part as it's much more tricky than the iOS part. I tried to implement changes from the fork mentioned above but so far failed to get it working as ReactNative parts seem to be outdated.

I also tried to embed it directly but I was not able to do that with the need for wrapping the JitsiMeetView in JitsiMeetActivity.

Notice that I temporarily changed the pubspec.yaml to relative import of jitsi_meet_wrapper_platform_interface. As the changes are in both packages I left it for testing purposes.

This is still work in progress, I will continue implementing the missing android part, but I'm looking forward for any feedback/suggestions/support as my android knowledge is quite limited.

Thank you

[predictron-cloud]

@kamskr, Did you have a chance to finish the android part? Maybe you have some version of android code that you can commit to some another branch and I can try to fix finish this work?

[kamskr]

@predictron-cloud

Unfortunately, I was not able to get it to work on Android. I don't have any progress that would be useful and is not already in this PR.