Add reusable workflows for out-of-tree extensions

[samansmink]

This pr

The goal is to provide reusable workflows that out of tree extensions can use to reduce code duplication in CI and make it easier for third party developers to maintain their extension CI.

This will allow us to

• remove most ci code from out-of-tree extensions (custom testing setups (e.g. http servers for testing) will need to remain)
• centrally maintain ci of out-of-tree extensions
• keep third party devs of extensions up to date with changes/additions to CI for extension binary testing and distribution.

Review/merge process

This pr has a companion pr in duckdb/extension-template#31. The process to merge the two is first this one, update the tag/branch in the extension-template PR to track current master, then merge extension-template. Finally, when v0.9.0 happens the tag for the workflow in extension-template and the duckdb version should be set to that.

[Mause]

Is there a particular reason we're storing them in this repo? We could have them in standalone repo's and make them official (and published to the github actions marketplace for example)

[samansmink]

@Mause Yea we thought about it, it was my plan initially. The problem is, to ensure toolchains/archs of extensions match the ones from the duckdb binary distributions, there needs to be a mapping of extension workflow version -> duckdb version. Without this mapping a developer distributing a duckdb extension will have no idea which workflow version to use for which duckdb version. The easiest/clearest way to users is by adding them to the duckdb repo so they always follow duckdb versioning.

Happy to discuss though if you disagree here

[Mytherin]

Thanks! LGTM