Make GroupCall work better with widgets

[robintown]

If the client uses a widget to join group calls, like Element Web does, then the local device could be joined to the call without GroupCall knowing. This adds a field to GroupCall that allows the client to tell GroupCall when it's using another session to join the call.

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Here's what your changelog entry will look like:

✨ Features

• Make GroupCall work better with widgets (#2935).

[RiotRobot]

The backport to staging failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-staging staging
# Navigate to the new working tree
cd .worktrees/backport-staging
# Create a new branch
git switch --create backport-2935-to-staging
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 79ccd7c330c2c8e84f175696fdeb692a341935c0
# Push it to GitHub
git push --set-upstream origin backport-2935-to-staging
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-staging

Then, create a pull request where the base branch is staging and the compare/head branch is backport-2935-to-staging.