Implement a blacklisting endpoint (instead of a blacklisting reactor and agent wrapper)

This issue has been migrated from [#8860](https://github.com/matrix-org/synapse/issues/8860).

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See https://github.com/matrix-org/synapse/pull/8821#issuecomment-736586382 for background on this.

Currently to apply an IP range blacklist in Synapse it is necessary to wrap the the reactor with `BlacklistingReactorWrapper` and then the agent with `BlacklistingAgentWrapper`. Ideally all of this logic could be encompassed in a custom `BlacklistingHostnameEndpoint` which would implement similar logic to `HostnameEndpoint`, but apply the blacklisting logic to it (the necessary logic is almost identical to what is currently in `_IPBlacklistingResolver`).

This should allow creating an agent with `Agent. usingEndpointFactory`.

Some things to note:

* This has to fit in somehow with the `ProxyAgent` (I suspect it should be an either or sort of thing, but not this should be further investigated).
* The separation of `SimpleHttpClient` and `MatrixFederationClient` makes this a bit more difficult.
* There's also some use of the `RedirectAgent` which may or may not be involved here.