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.

Pull Request Checklist

• [x] Pull request is based on the develop branch
• [x] Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• [x] Code style is correct (run the linters)

[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: https://github.com/element-hq/synapse/blob/aab3672037ee9ea71c54c89c86d9c0cc35ae184f/synapse/config/registration.py#L37-L40

[CLAassistant]

All committers have signed the CLA.

[MadLittleMods]

Thanks for continuing down this path @V02460 :slightly_smiling_face: