MSC4108: Mechanism to allow OAuth 2.0 API sign in and E2EE set up via QR code - 2024 version of rendezvous

Tracking issue for the [2024](https://github.com/matrix-org/matrix-spec-proposals/blob/87f8317a902cd7bc5c2d2d225f71021b3a509e2d/proposals/4108-oidc-qr-login.md) version of the rendezvous session from MSC4108.

## Context

This is the implementation for the secure rendezvous session part of the 2024 version of [MSC4108](https://github.com/matrix-org/matrix-spec-proposals/pull/4108) which allows for Sign in with QR feature.

> [!IMPORTANT]
> This experimental feature is being superseded by an a feature tracked by https://github.com/element-hq/synapse/issues/19434. But it is recommended to stay with this current version until MSC4108 is finalised.

## Current design in Synapse

It is implemented as Twisted servlets in Rust.

### Usage

Pre-requisites:
- OAuth 2.0 API needs to be enabled either via the `matrix_authentication_service` or the `experimental_features.msc3861` configuration options

The feature is configured using:

```
experimental_features:
msc4108_enabled: true
```

or

```
experimental_features
msc4108_delegation_endpoint: https://example.com/rendezvous
```

## Caveats to be aware of

- Limitations of the MSC:
- the use of `ETag` headers can cause issues with proxies and load balancers. e.g. https://github.com/element-hq/synapse/issues/18155
- additional CORS headers may need to be allowed on proxies/load balancers
- it is not tolerant to server and client clocks being out of sync: https://github.com/matrix-org/matrix-js-sdk/issues/5141
- Synapse implementation:
- in progress rendezvous sessions are dropped during a server restart
- currently restricted to the main process
- no specific rate limit controls
- maximum session duration is not configurable
- the maximum concurrent sessions is not configurable

## How do we know when to remove this

We can remove support for this once we land support for whatever the final version of MSC4108 ends up being + some time period to allow clients to catch up.