Add support for SQL functions

[electrum]

Description

Add support for inline SQL functions and for persistent SQL functions with CREATE FUNCTION and DROP FUNCTION. Functions can be stored using the Memory or Hive connectors.

Additional context and related issues

Documentation: #18854

Release notes

(x) Release notes are required, with the following suggested text:

# General
* Add support for inline SQL functions. ({issue}`19308`)
* Add support for `CREATE FUNCTION` and `DROP FUNCTION`. ({issue}`19308`)

# CLI
* Handle functions when splitting statements. ({issue}`19308`)

[martint]

Some initial comments for the first commit.

[electrum]

I split the first three commits out into #19508

[dain]

I split the inline functions commit to #19521

[dain]

Last two commits look good