GH-50223: [C++][Compute] Support string_view/binary_view keys in the hash-aggregate Grouper

[fangchenli]

Rationale for this change

Grouper rejected string_view and binary_view group keys with NotImplemented: Keys of type string_view, so Table.group_by on a view-typed key failed.

What changes are included in this PR?

• Add BinaryViewKeyEncoder in row_encoder_internal.{h,cc}. It encodes view keys using the existing variable-length row format and decodes them into a fresh view array of the original type.
• GrouperImpl::Make dispatches view keys to the new encoder.
• GrouperFastImpl::CanUse rejects view keys, so they fall back to GrouperImpl.

Are these changes tested?

Yes, unit tests added.

Are there any user-facing changes?

Yes. Table.group_by now accepts string_view and binary_view group keys

• GitHub Issue: [C++][Compute] Support string_view/binary_view keys in the hash-aggregate Grouper #50223

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[github-actions]

⚠️ GitHub issue #50223 has been automatically assigned in GitHub to PR creator.