[FEATURE]: Make JitsiMeet methods non-static

[tuantvu]

Description

Static methods are not conducive to mocking in unit test. Also, the idea of global listeners vs per meeting listeners bloat the code and make it complex.

Is the feature supported by Jitsi?
jitsi_meet Flutter plugin is dependent on the Android and iOS SDK from jitsi.org. If those SDKs do not support a feature, we also can not add it.

• Yes: Paste link to Jitsi documentation here
• No
• N/A

Describe the solution you'd like

Enable clients to instantiate JitsiMeet like final jitsiMeet = JitsiMeet(). Test if more than one JitsiMeet conference can even be opened at the same time. If not, maybe implement a singleton like JitsiMeet.instance.

Describe alternatives you've considered

Additional context