feat(external-api) Add endConference command

[shawnchin]

Command to trigger same action as that accomplished by the new "End Meeting for all" button.

Without this command, it will be difficult to write effective click handlers for 'end-meeting' (introduced in https://github.com/jitsi/jitsi-meet/pull/12179).

[jitsi-jenkins]

Hi, thanks for your contribution! If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[damencho]

Hey @saghul will you manage to look at it next week, thanks?