3Delight multipart render support

[gkocov]

Generally describe what this PR will do, and why it is needed

• Support for rendering to multipart EXRs by using the same file name output parameter across multiple outputs.
• Recreated PR against 1.4_maintenance branch.
• Addressed all the notes from the discussion in the main branch PR - 3Delight multipart render support #5844

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Thanks!