Call guest access link creation to join calls as a non registered user via the EC SPA

[toger5]

This adds a button to the room header when there is an ongoing call in a public room. Clicking it will gernerate a external EC link that allows to join this call with a element call SPA and a "guest" account on the homeserver configured on the spa.

The homeserver for the SPA "guest" accounts is part of the SPA configuration. A new configuration parameter guest_spa_url is part of the EW element-call configuration to setup a custom SPA for share links.

The share button will be shown:

• if the room is public (join without invites)
• and if EW has a guest_spa_url configured

Screenshots

In the lobby: When clicked: During call:

Checklist

• [x] Tests written for new code (and old code if feasible)
• [x] Linter and other CI checks pass
• [x] Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Call guest access link creation to join calls as a non registered user via the EC SPA (#12259). Contributed by @toger5.

[t3chguy]

@toger5 can you please update the PR title to provide more context in the changelog