Add a method for Synapse modules to make federation requests over HTTP

This issue has been migrated from [#15149](https://github.com/matrix-org/synapse/pull/15149).

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Provides a fairly basic interface for Synapse modules to complete HTTP federation requests in the form of a new `send_federation_http_request` module API method. The motivation behind this change is to allow a Synapse module to query the [`POST /_matrix/federation/v1/publicRooms`](https://spec.matrix.org/v1.6/server-server-api/#post_matrixfederationv1publicrooms) federation endpoint of one or more remote homeservers.

An abstraction layer (error types, etc.) over Synapse's `MatrixFederationHttpClient` was used in order to prevent having to stabilise any internal federation handling code, and to provide a simpler interface for modules to use.

TODO: unit tests

Start by reviewing the proposed send_federation_http_request module API and the MatrixFederationHttpClient abstraction described in the issue. Add the missing unit tests, including a request to the publicRooms federation endpoint and the documented error-handling behavior; done means the module-facing interface and its error types are covered.

Written by the indexing model from the issue text.