Add --no-secrets-in-config command line option (#18092)

Adds the `--no-secrets-in-config` command line option that makes Synapse
reject all configurations containing keys with in-line secret values.
Currently this rejects

- `turn_shared_secret`
- `registration_shared_secret`
- `macaroon_secret_key`
- `recaptcha_private_key`
- `recaptcha_public_key`
- `experimental_features.msc3861.client_secret`
- `experimental_features.msc3861.jwk`
- `experimental_features.msc3861.admin_token`
- `form_secret`
- `redis.password`
- `worker_replication_secret`

> [!TIP]
> Hey, you! Yes, you!  If you think this list is missing an item,
please leave a comment below. Thanks :)

This PR complements my other PRs[^1] that add the corresponding `_path`
variants for this class of config options. It enables admins to enforce
a policy of no secrets in configuration files and guards against
accident and malice.

Because I consider the flag `--no-secrets-in-config` to be
security-relevant, I did not add a corresponding `--secrets-in-config`
flag; this way, if Synapse command line options are appended at various
places, there is no way to weaken the once-set setting with a succeeding
flag.

[^1]: [#17690](https://github.com/element-hq/synapse/pull/17690),
[#17717](https://github.com/element-hq/synapse/pull/17717),
[#17983](https://github.com/element-hq/synapse/pull/17983),
[#17984](https://github.com/element-hq/synapse/pull/17984),
[#18004](https://github.com/element-hq/synapse/pull/18004),
[#18090](https://github.com/element-hq/synapse/pull/18090)


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