Rewrite `ShowCharset` node

The `ShowCharset` node seems to just be a `SubqueryAlias`-esque wrapper around `information_schema.character_sets`. It breaks a lot of conventions we have around how nodes are structured, and its debug string is effectively a blackbox so it was hard to debug when a filter was pushed down through it.

It might make sense to structure and build `ShowCharset` to be more similar to other `SHOW` nodes and to resolve/fetch `information_schema.character_sets` during rowexec.

Start by comparing the ShowCharset node with other SHOW nodes and the SubqueryAlias pattern, then trace how information_schema.character_sets is resolved and fetched during rowexec. Done means ShowCharset follows the project's node conventions, has a useful debug string, and preserves filter pushdown behavior.

Written by the indexing model from the issue text.