Add the Ability to Specify Region and Signer Override in S3 SDK

[original-brownbear]

Some S3 compatible blob stores force v4 authentication with regions other than the default us-east-1 or don't support v4 authentication.
Currently there is no way of changing the signing region (we only allow setting the endpoint) or forcing the S3 SDK to not use v4 auth if the endpoint does not support it.

We should bring back the region setting to allow v4 auth to properly work if the blobstore supports it.
Also, we should expose the signer override setting to allow forcing the supported auth mechanism in case the SDK is unable to determine the correct one for a given endpoint.

Link: https://discuss.elastic.co/t/s3-snapshot-on-s3-compatible-servers/217750/4

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)