serve well-known client and server config

This way users can have a simpler time configuring this stuff and we can worry about the spec compliance parts and specifying the same thing over and over parts.

New configuration options

Naming rationale

• [server_discovery] is so named because the spec uses that phrase:
  • https://spec.matrix.org/v1.11/client-server-api/#server-discovery
  • https://spec.matrix.org/v1.11/server-server-api/#server-discovery
• [server_discovery.client] and [server_discovery.server] are so named because they configure the client and server respectively. I could've gone for client_server and server_server but that feels needlessly verbose.
• authority is the actual authority for client-server and server-server traffic.
• insecure (boolean) is pretty self-explanatory I think.

Examples

Delegate requests to an alternate domain:

server_name = "computer.surgery"

[server_discovery]
client.authority = "matrix.computer.surgery"
server.authority = "matrix.computer.surgery"

Same as before but also make federation go over 443 instead of 8448:

server_name = "computer.surgery"

[server_discovery]
client.authority = "matrix.computer.surgery"
server.authority = "matrix.computer.surgery:443"

The following can be done for local development to use HTTP instead of HTTPS for the client configuration:

[server_discovery]
client.insecure = true

To enable sliding sync, you must opt in with an additional option, because this sliding sync impl is buggy:

[server_discovery]
client.advertise_buggy_sliding_sync = true