Reusable workflows + vcpkg in template

[samansmink]

Note: draft pr for reviewing purposes.

PR goals

• integrate vcpkg into extension template
• use reusable workflows
• switch to using loadable extensions for client tests

vcpkg dependency in example extension

OpenSSL is now integrated in quack example extension to test/demonstrate vcpkg. What this means is that to build the example you can no longer "just build it", instead you need to either:

• Use VCPKG
• have OpenSSL installed already
• remove the openssl part manually before building

reusable workflows

The goal here is to no longer have all extensions have their own CI config, instead relying on remote workflows that is managed by us and can be updated to ensure all extensions can easily keep their CI up to date.

The main question here is, where should these workflows live? in a separate extension? in duckdb main repo? In the extension template itself? I think having them live either in a separate repo or in duckdb/duckdb makes the most sense.

[carlopi]

Partial review, will finish the rest in a bit, but looks strong.

[samansmink]

This needs more work, I'll close this one for now