feat(pg_catalog): stub pg_opclass

[kwannoel]

I hereby agree to the terms of the Singularity Data, Inc. Contributor License Agreement.

What's changed and what's your intention?

stub pg_opclass to support sqlancer.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)
• Try out sqlancer, it should work with this branch.
  Update: More system tables needed, see: Tracking: Introduce Sqlancer #3364 (comment). Does not work for now.

Documentation

Types of user-facing changes

Note that although pg_opclass is user-facing. We do not want to expose it to end user. It is just introduced for sqlancer support.

Release note

Refer to a related PR or issue link (optional)

#3431 (comment)

[codecov]

Codecov Report

Merging #6167 (d4c3712) into main (a878685) will decrease coverage by 0.29%.
The diff coverage is 32.27%.

@@            Coverage Diff             @@
##             main    #6167      +/-   ##
==========================================
- Coverage   74.65%   74.36%   -0.30%     
==========================================
  Files         931      935       +4     
  Lines      149324   150096     +772     
==========================================
+ Hits       111471   111612     +141     
- Misses      37853    38484     +631     

Flag	Coverage Δ
rust	74.36% <32.27%> (-0.30%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/common/src/catalog/internal_table.rs	100.00% <ø> (ø)
src/frontend/src/catalog/catalog_service.rs	5.42% <0.00%> (-0.36%)	⬇️
src/frontend/src/catalog/mod.rs	75.67% <ø> (ø)
src/frontend/src/catalog/view_catalog.rs	0.00% <0.00%> (ø)
src/frontend/src/handler/create_view.rs	0.00% <0.00%> (ø)
src/frontend/src/handler/drop_database.rs	80.51% <0.00%> (ø)
src/frontend/src/handler/drop_schema.rs	68.29% <0.00%> (ø)
src/frontend/src/handler/drop_view.rs	0.00% <0.00%> (ø)
src/frontend/src/handler/handle_privilege.rs	78.60% <0.00%> (+1.43%)	⬆️
src/frontend/src/observer/observer_manager.rs	0.00% <0.00%> (ø)
... and 81 more

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