Fix S3 Select payload join

[TAKEDA-Takashi]

The original code has a potential bug that decodes fail if the payload is split in the middle of a multibyte character.
This can be avoided by joining and then decoding.

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[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest master or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

Can you please add regression test for that ?

Do I understand correctly that it fixes the problem where the records split is in the middle of multi-byte character? If so it should be fairly easy to add unit test for.

[TAKEDA-Takashi]

Added commit.
The actual payload is about 65KB, which is long data, but the test data is short.

[potiuk]

Perfect! Thank you!