Add PRQL extension

[ywelsch]

The PRQL extension (see https://github.com/ywelsch/duckdb-prql) is based on an older copy of the extension template. It might need to be brought up-to-date first for this to work?

[ywelsch]

The PRQL extension needs the PRQL transpiler, which is written in Rust. The build is currently failing because the Rust toolchain is absent in this build environment.

[samansmink]

@ywelsch ah got ya, we will need to get that in first, i can take a stab at that. It's basically all there in the delta extension

[rustyconover]

I have a fix in my repo for the linux_arm64 cross compile.

[carlopi]

@ywelsch (& also @rustyconover, given you are here and the comment is the same): I think on main I have the fix for the documentation test failing.

Could you merge or rebase on latest main? Thanks! Then extensions should pass.

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Independent, we can also change this later, I think in both case it would be nice to have it explicit from the descriptor that this is a C++ + Rust extension. I think probably the simpler is having the language as C++ & Rust, would that make sense? That would allow also to enable branching on including Rust toolchain or not (see comment here: #22 (review))

[samansmink]

I think we can just merge this first, fix the docs thing later

[carlopi]

Actually, moving forward, congrats @ywelsch!

[ywelsch]

I've just pushed another update :)

[samansmink]

okay lets just let it run then merge both PRs