Persist the ACME account key: dns01_txt mode creates a new Let's Encrypt account per certificate

`createLegoClient` ([externalcas/external.go:96](https://github.com/esnet/acme-proxy/blob/6ff550aab9bcfd0c78474bf531e114305186e337/externalcas/external.go#L96)) generates a fresh account key on every call, and nothing persists it. In `dns01_txt` mode, [external.go:148](https://github.com/esnet/acme-proxy/blob/6ff550aab9bcfd0c78474bf531e114305186e337/externalcas/external.go#L148) then does a plain `Registration.Register`, so every certificate issued through Let's Encrypt creates a new ACME account.

This has two consequences:

1. **Rate limits.** Let's Encrypt allows only [10 new accounts per IP per 3 hours](https://letsencrypt.org/docs/rate-limits/).
2. **Revocation should fail.** `RevokeCertificate` also builds a fresh client, so it signs with an account that never issued the certificate. [RFC 8555 section 7.6](https://www.rfc-editor.org/rfc/rfc8555#section-7.6) does not allow that.

This issue could be solved by adding an optional `account_key_path` under `authority.config` that gets loaded/generated on startup.
This way, registration would be idempotent, because `newAccount` with a known key returns the existing account ([RFC 8555 section 7.3](https://www.rfc-editor.org/rfc/rfc8555#section-7.3)).

Start in externalcas/external.go at createLegoClient (line 96), the dns01_txt registration path (line 148), and RevokeCertificate. Trace how authority.config is loaded and how clients are created at startup; done means an optional account_key_path loads or generates one, registration reuses the account, and revocation uses the issuing account.

Written by the indexing model from the issue text.