[function] refine file io connector

[nlu90]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #

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Motivation

1. FileListingThread doesn't need to do the permission check since the config.validate() has already done it. Also, as a ListingThread, it doesn't need the Write permission on the directory.
2. write permission is needed only when keepFile option is set to false.

Modifications

1. only do file writable permission check when keepFile is false.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

Documentation

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[nlu90]

/pulsarbot run-failure-checks

[nlu90]

@freeznet Could you help take a look?

[nlu90]

/pulsarbot run-failure-checks