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

[matrixbot]

This issue has been migrated from #8860.

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See matrix-org/synapse#8821 (comment) 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.