adapter: Changes to get aws_external_connection_role from args

[moulimukherjee]

Adding an argument to accept a Materialize Role ARN in environmentd and setting it in AwsPrincipalContext to be used later for AWS connections #23282

Motivation

Split this PR from #23282, so that this can be merged by 0.79 in case the other one takes longer (and unblock cloud so that they can start passing the materialize role arn).

Corresponding cloud issue: https://github.com/MaterializeInc/cloud/issues/8191

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

cc @jubrad