This commit adds external test modules. These are modules meant for
external systems to test edge cases in elasticsearch, but only within
snapshots. They are not meant to be used in production, so protections
are also added from their accidental inclusion in release builds.
Note that this commit does not actually add any new modules, it only
adds the infrastructure for the new modules, under
`test/external-modules`.
This solves half of the problem in #46813 by moving the S3
tests to using the shared minio fixture so we at least have
some non-3rd-party, constantly running coverage on these tests.
Ryan Ernst
Tanguy Leroux
Armin Braun