postgresql_extensible: query.tagvalue is strangely a CSV rather than a TOML array

### Use Case

When creating custom queries `tagvalue` is widely used to define result columns to be used as tag. Given that whole config file is a TOML syntax, use of CSV string format for `tagvalue` feels less natural than what TOML array would be.

### Expected behavior

Supporting 2 types for the same config param can be messy, so I propose deprecate `tagvalue` and introduce `tagvalues` parameter with array type.

### Actual behavior

`tagvalue` is a CSV string.

### Additional info

_No response_

Start by locating the postgresql_extensible implementation and its configuration handling and tests. Review how the existing tagvalue CSV parameter is parsed, then determine the compatibility and deprecation behavior for a new tagvalues TOML array; done means the new parameter works and the existing behavior is addressed consistently.

Written by the indexing model from the issue text.