Extend postgres_indexes view to exclude expression indexes from reindexing

What does this MR do?

This adds two columns to postgres_indexes:

1. expression (for an expression index)
2. partial (for a partial index)

This allows us to (temporarily) disable reindexing for expression indexes until we have a fix for #272997 (closed).

The reason I'm doing this is that I'd like to re-enable reindexing in staging as soon as possible, so that we get more feedback from it.

Also adding the partial column here, even we don't necessarily need it. I think this will be handy for us to reason about indexes manually.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content