Move I/O to executor thread pool

[cyrosy]

Breaking Change:

The images passed to the payload must be within a whitelisted external dir

Description:

Change requested by @MartinHjelmare on my previous PR #23523

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly (example).
• New dependencies have been added to requirements in the manifest (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

[amelchio]

While you are at it, I think this should probably check whitelist_external_dirs ?

[cyrosy]

@amelchio check added, I edited the PR to add a BC description : old payload with images outside of whitelist_external_dirs would not work as before

[MartinHjelmare]

Good! Just move the import.

[MartinHjelmare]

Can be merged when build passes.