Improve FIPS mode

https://www.erlang.org/doc/apps/crypto/fips.html

1. The first step is to enable FIPS mode when compiling Erlang. That merely allows toggling it on / off if the underlying crypto library (OpenSSL) supports it. For our packaging we can do that here https://github.com/apache/couchdb-ci/pull/44.

2. Implement a runtime FIPS mode toggle. Currently we can enable it at compile time, however, that requires having two builds, two package flavors etc., Instead, use a runtime config option to toggle it. In [couch_hash](https://github.com/apache/couchdb/blob/main/src/couch/src/couch_hash.erl#L17-L45), to avoid a performance impact, use a [persistent term](https://www.erlang.org/doc/man/persistent_term.html) to minimize config ETS lookups if performance degradation is of concern. (Implemented in https://github.com/apache/couchdb/pull/4443)

3. FIPS mode, when enabled is already exposed in the [features list](https://github.com/apache/couchdb/blob/main/src/couch/src/couch_server.erl#L277-L283).

4. Add comments around each md5 usage explaining that it's not for cryptographic use.

5. Any places where TLS is used, would potentially need to reduce the [list of advertised algorithms / curves / parameters](https://esl.github.io/MongooseDocs/latest/developers-guide/OpenSSL-and-FIPS/) to conform to the FIPS approved list. If that's not done it would lead to `unsupported` crashes and exception during attempted use. A few places that might happen:
- chttpd (mochiweb) TLS support
- couch_replicator (ibrowse)
- Erlang intra-node TLS

Review the Erlang FIPS documentation and the remaining scope after couchdb/pull/4443. Inspect couch_hash.erl, couch_server.erl, and the chttpd, couch_replicator, and intra-node TLS areas, then locate MD5 uses. Done means runtime FIPS behavior, non-cryptographic MD5 uses documented, and TLS options conforming to the approved set without unsupported failures.

Written by the indexing model from the issue text.