Better handling of appservices in user directory

This issue has been migrated from [#11052](https://github.com/matrix-org/synapse/issues/11052).

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Synapse has excluded appservice users from the directory for ages. It hasn't excluded appservice senders (but I'm about to make this change to fix #11025). This means there's a whole class of virtual users who we might be able to see but not reach out to.

Ideally appservices would be able to specify if their virtual users are contactable, or some other flag that means they should be exposed in the user directory. They should probably also be able to specify if the appservice sender is contactable too.

The existing rules (you can only see users in public rooms, or in private rooms you share with them) should still apply.

See also discussions https://github.com/matrix-org/synapse/pull/10914#discussion_r716859548 and https://github.com/matrix-org/synapse/pull/11026#discussion_r725252925