feat: add PostgresRowSequence.getColumns() to get column metadata

[SteveLauC]

This commit implements PostgresRowSequence.getColumns() to enable users to retrieve column metadata of their query results.

Discussion: #576

Closes #576

[SteveLauC]

• Based on this comment, RowDescription.Column should stay private, and a new type PostgresColumn is added. This type and its fields are public, but the fields are immutable, and the init() function is private. I did this because I want it to behave like an accessor: users are only allowed to construct it via the getColumns() method, once created, users can only read it.

• API naming: I would like to directly use columns(), but the compiler complains that there is already a field with the same name. So I go with getColumns().

• Definition of this new PostgresColumn type: I blindly use the fields of RowDescription.Column

• Two unit tests are added

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Just my two cents — I'm happy to adjust based on the maintainer's feedback. :)