Add support for custom credential loader for S3 FileIO

[phillipleblanc]

Which issue does this PR close?

• Closes Add ability to specify a custom AWS credential loader for S3 FileIO #1527

What changes are included in this PR?

Adds the ability to provide custom extensions to the FileIOBuilder. Currently the only supported extension is CustomAwsCredentialLoader which is a newtype around AwsCredentialLoad, which is what OpenDAL expects.

I've added extensions to the RestCatalog as well, and when its constructing FileIO for table operations, passes along any defined extensions into the FileIOBuilder, which then get passed into the underlying OpenDAL constructors.

Are these changes tested?

Yes, tests added in crates/iceberg/tests/file_io_s3_test.rs that verify the extension is working.

[Xuanwo]

Thank you @phillipleblanc for working on this. Ideally, we should have this support built into reqsign or opendal, but I'm happy to merge this PR to give our users access.

[liurenjie1024]

This is another motivating case of #1314

[phillipleblanc]

This is another motivating case of #1314

Ah yeah, I wasn't aware of that discussion - thanks for the link! I represent another case where I'm already heavily invested in the object_store ecosystem and would like to be able to use that to power my Iceberg reads/writes in DataFusion.