Add recaptcha_{private,public}_key_path config option

[V02460]

Another config option on my quest to a *_path variant for every secret. Adds the config options recaptcha_private_key_path and recaptcha_public_key_path. Tests and docs are included.

A public key is of course no secret, but it is closely related to the private key, so it’s still useful to have a *_path variant for it.

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[MadLittleMods]

We can dumb this down a bit:

Suggested change

	You have configured both `recaptcha_private_key` and
	`recaptcha_private_key_path`. These are mutually incompatible.
	You have configured both `recaptcha_private_key` and
	`recaptcha_private_key_path`. Only one or the other can be specified at a time.

[MadLittleMods]

This is a slight variation on the typical error message we've been using for this:

Conflicting options 'macaroon_secret_key' and 'macaroon_secret_key_path' are
both defined in config file.

I like the version that this PR has but we should probably be consistent and update all the other similar errors.

[V02460]

@MadLittleMods: Could you clarify what you want the error message to be?

For these PRs I did not use my own wording, but took it from registration_shared_secret:

synapse/synapse/config/registration.py

Lines 37 to 40 in aab3672

	CONFLICTING_SHARED_SECRET_OPTS_ERROR = """\
	You have configured both `registration_shared_secret` and
	`registration_shared_secret_path`. These are mutually incompatible.
	"""