fix: reintroduce FetchFieldFromStruct as a public UDF

[agavra]

Description

When upgrading from 5.3->5.4 commands that were already written into the command topic would fail with "Cannot find function FETCH_FEILD_FROM_STRCUT". The reason is that we made structs stop rewriting with that UDF and instead inlined it's usage and prevent people from using that function externally. This re-introduces the change into 5.4. so that upgrades are possible.

NOTE: This will not be merged to master and master will be considered a breaking upgrade from 5.4

Testing done

• verified the problem by running 5.3 and issuing a command with struct access
• upgraded to 5.4 and checked if it can recover; before this fix it cannot and after it can

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[apurvam]

Here is the PR which removed FETCH_FIELD_FROM_STRUCT #3639

[apurvam]

This looks good to me overall The handling of FETCH_FIELD_FROM_STRUCT looks totally special cased though. Would be good to understand why that is the preferred approach.

[agavra]

@apurvam - it was always special cased because unlike normal UDFs, FETCH_FIELD_FROM_STRUCT does not have a deterministic return type. The code introduced in this PR is manually cherry-picked (inverse) from #3639 (plus some additional tests)

[apurvam]

LGTM, thanks!

[stevenpyzhang]

LGTM thanks for the fix @agavra