storage/copy-to-s3: Require format option

[rjobanp]

Motivation

• This PR fixes a recognized bug. Fixes storage/s3: FORMAT option should be required or default to supported value #27101

Tips for reviewer

This leaves the default as TEXT for all other COPY statements, but requires the FORMAT option is provided for the copy-to-expr statement.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• 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:

[maddyblue]

Hmm not sure how I feel about this from a SQL perspective because it differs from the postgres default. Maybe instead of defaulting to CSV for that variant, require that it be set to something (i.e., no default)? SQL council should weigh in.

[morsapaes]

+1 making it a required option when the AWS CONNECTION option is specified.

[benesch]

+1 making it a required option when the AWS CONNECTION option is specified.

+1 from me too.

[rjobanp]

Updated to make required instead of defaulted for COPY TO <expr>