Add --unbuffered mode

[rolandwalker]

Description

Per https://pymysql.readthedocs.io/en/latest/modules/cursors.html#pymysql.cursors.SSDictCursor

Unbuffered Cursor, mainly useful for queries that return a lot of
data, or for connections to remote servers over a slow network.

Instead of copying every row of data into a buffer, this will fetch
rows as needed. The upside of this is the client uses much less
memory, and rows are returned much faster when traveling over a slow
network or if the result set is very big.

Fixes #1184.

Checklist

• I've added this contribution to the changelog.md.
• I've added my name to the AUTHORS file (or it's already there).
• I ran uv run ruff check && uv run ruff format && uv run mypy --install-types . to lint and format the code.