(1) Implement generic oak_cache (#1298)

Part of #1234

Part of a series of PRs to get the sources mechanism integrated with
oak. Most of the PRs revolve around restructuring the cache in a way
that I think makes a lot more sense (particularly if we want it to be
usable elsewhere).

We end up with:
- `oak_cache` - Generic `Cache` mechanism
- `oak_srcref` - Home to `SrcrefCache`, requires R, extracts the
`srcref` attribute
- `oak_source` - Home to `SourceCache`, which knows how to download CRAN
R package tarballs and R version tarballs

And `oak_sources` gets removed at the end.

When I worked on the caching stuff earlier, I struggled to reconcile
with the fact that different caching sources return different things. We
have 3 things we can return:
- srcrefs, which is just the reconstructed `R/` folder
- CRAN R package tarballs, which is a single R package in its entirety
- CRAN R version tarballs, which is a single R version in its entirety

Rather than trying to squish these together into a single cache
mechanism, which required us to drop useful things from the CRAN
downloads to align with the more minimal result we get from the srcref
extractor, we now keep them separate in separate caches, so we end up
with:

- `oak_srcref::SrcrefCache` feeding to `.cache/oak/srcref/`
- `oak_source::SourceCache::cran` feeding to `.cache/oak/source/cran/`
- `oak_source::SourceCache::r` feeding to `.cache/oak/source/r/`

Each cache folder now has a consistent structure that also maximizes
information retention. i.e. in the CRAN download we keep the entire R
package source, including things like `man/`, `src/`, and any `inst/`
folder you might have included. I imagine this could be useful in the
future (navigating to C functions?).

I'm also hoping that `oak_source` can be useful on its own without
requiring the heavier R dependency that `oak_srcref` has.

The end result of this series of 5 PRs is a production level
`SourceHandler` that is integrated in the LSP that pulls sources using
`oak_srcref` and `oak_source` when you are in a debug build. Release
builds still don't have it turned on yet. And it isn't being utilized in
any LSP features yet. I'll turn them on one by one after merging this
series.

Author: DavisVaughan

Cargo.lock

@@ -75,6 +75,7 @@ log = "0.4.29"
 mime_guess = "2.0.5"
 nix = { version = "0.31", features = ["fs", "poll", "process", "signal"] }
 notify = "8.2.0"
+oak_cache = { path = "crates/oak_cache" }
 oak_core = { path = "crates/oak_core" }
 oak_db = { path = "crates/oak_db" }
 oak_fs = { path = "crates/oak_fs" }

Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "oak_cache"
+version = "0.1.0"
+authors.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+
+[dependencies]
+anyhow.workspace = true
+etcetera.workspace = true
+log.workspace = true
+oak_fs.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true
+
+[lints]
+workspace = true