Don't enable ES|QL MD5 function in FIPS mode

[tvernum]

MD5 should not be used in a FIPS mode JVM, except for in certain older TLS ciphers that depend on it.

Because those TLS ciphers may need it, it technically works (at least in FIPS 140-2) but is not compliant.

Some FIPS-mode deployments may configure the JVM to disable MD5 entirely (because they do not need or enabled those older ciphers).

We have two options, either:

1. Check FIPS mode and disable MD5. This should be possible because the FIPS setting is in X-Pack core, and the esql module already depends on that.
2. Just check for an error when loading the MD5 MessageDigest and disable the function.

The former would mean ES|QL's MD5 was strictly unavailable in FIPS mode. The latter would mean that we allow anyone to turn of MD5 if they wish.

I think I would be happy with either or both.

Relates: #118938

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Pinging @elastic/es-security (Team:Security)

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[tvernum]

I've opened

• Handle unavailable MD5 in ES|QL #130158

On reflection, I decided that the only acceptable behaviour is

Just check for an error when loading the MD5 MessageDigest and disable the function.

There are two reasons for taking that direction:

1. Strictly speaking, FIPS 140 does not forbid the use of MD5. It's simply that it is not an acceptable cryptographic hash. It is allowable to use it for non cryptographic purposes, just like we use murmur3 as a non-cryptographic hash inside Elasticsearch. If an ES|QL user is using a FIPS mode ES cluster and wants to calculate MD5 hashes for non cryptographic purposes that's OK. Removing the MD5 function on FIPS would be a breaking change without justification. And, since ES|QL has a general hash( algorithm, data ) function, turning off the md5 function wouldn't stop people from using MD5 if it was available in their JVM
2. In Java 14 the specification for MessageDigest was changed so that MD5 is no longer required. That is, any JDK (FIPS or non-FIPS) is free to drop MD5 support if they wish. So it would be silly for us to disable the md5 ES|QL function in FIPS but require it in non-FIPS mode when the JRE no longer provides that guarantee.