[Admin] Make prepared statements queryable with SQL

[levkk]

Allow the user to send a SELECT statement to interact with the prepared statements view (SHOW PREPARED).

Example

SELECT * FROM show_prepared() ORDER BY memory_used;

This we want to support:

1. selecting specific columns
2. selecting all columns
3. ordering by 1 or more columns (ORDER BY clause)
4. filtering by 1 or more columns (WHERE clause)

Build this as a generic "query engine" inside admin and apply to SHOW PREPARED. Follow up issues will be to apply it to all other views.

[rimrakhimov]

Hey @levkk, may I give it a try or are you going to do this in-house?

I don't have a thorough plan yet, but there are draft steps I've come up with by skimming the codebase:

1. If the query starts with SELECT, parse it as usual SQL statement, validate and handle execution to the "query engine".
2. Build the resultant rows with all column values similar to how ShowPreparedStatements::execute does right now
3. Apply filters, ordering, projection and return the result. For ordering there is a Buffer::sort method which could be applicable here as well, but I'm not sure if Buffer logically fits the logic outside of aggregating multiple shards. Maybe the method implementation can be extracted somewhere and reused between Buffer and some other struct.