feat: support for simple extensions dependencies

[jvanstraten]

No description provided.

[jvanstraten]

This one has been on my mental TODO list for a while now: it's currently impossible for an extension to define functions that make use of types from another extension. Some problems with that:

• The Substrait core extensions are split over a whole bunch of files because, well, it'd be an absolute mess otherwise, but consumers that define their own extension types currently can't follow the same paradigm for any function that uses any extension type.
• Consumer X may want to make use of the types defined by consumer Y for extra compatibility. For example, Arrow is/will be defining extension types for all the Arrow types that don't exist in Substrait core. Execution engines besides Arrow/Acero may want to define functions for those types as well, if they use Arrow's type system internally.

AFAICT, the only missing link for supporting this as far as the specification is concerned is the lack of a way to define and refer to extension URI dependencies in YAML, so that's what this PR adds.

[jacques-n]

Seems reasonable. Do you want to also write up some markdown for this?

[jvanstraten]

Yep, good point.

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[jacques-n]

Is this a valid URI?

[EpsilonPrime]

It is a valid URI reference which we tend to accept in implementations. To make this a better example we could just prepend file:.

[EpsilonPrime]

I've made that change.