Download Flux modules within runtime as part of Salsa query

[nathanielc]

Now that we can incrementally compile the stdlib (see #5135) next steps are to dynamically download and compile Flux modules.

This issue is about adding the ability for the Rust side to download Flux modules using the module API (see #5224).

Some implementation notes:

• We plan to reuse the token from the query request when making the Flux modules API request. This means passing the token from Go to Rust
• Downloading the module will be part of the source() salsa query (see https://github.com/influxdata/flux/blob/master/libflux/flux-core/src/db.rs#L57).
• We do not need to worry about WASM/LSP as part of this issue. We may need an abstraction in the future to download modules but that can wait until we better understand how the LSP/WASM intend to download modules.

Version selection (i.e. finding the max of mins) is out of scope for this issue. The behavior will be to always download and compile the latest version of the module.

NOTE: Salsa DB might need to differentiate between modules from different orgs. Or we create a new salsa DB per request to avoid any cross org modules.

DOD:

• Flux modules can be downloaded from the registry and dynamically compiled
• Support for downloading latest version of a module.
• Flux scripts work with modules in tests
• Full integration testing is a separate issue

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