MSC3244 room capabilities implementation

[t3chguy]

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
• Pull request includes a sign off
• Code style is correct (run the linters)

Example response:

{
    "capabilities": {
        "m.room_versions": {
            "default": "6",
            "available": {
                "1": "stable",
                "2": "stable",
                "3": "stable",
                "4": "stable",
                "5": "stable",
                "6": "stable",
                "7": "stable",
                "org.matrix.msc2176": "unstable",
                "org.matrix.msc3083": "unstable"
            },
            "org.matrix.msc3244.room_capabilities": {
                "knock": {
                    "preferred": "7",
                    "support": [
                        "7"
                    ]
                },
                "restricted": {
                    "preferred": null,
                    "support": [
                        "org.matrix.msc3083"
                    ]
                }
            }
        },
        "m.change_password": {
            "enabled": true
        }
    }
}

[clokep]

Note that complement failing is unrelated to this PR, see #10275.

[richvdh]

for links: this implements MSC3244.

[richvdh]

lgtm