Support overriding the "Spin data dir" via env var

[lann]

The Spin "data dir" (e.g. ~/.local/share) is used as the root for plugin and template state. We have a request (part of #2555) to make this customizable to aid integration with other development tooling.

The data dir is constructed here:

spin/crates/common/src/data_dir.rs

Line 7 in aedc2b8

pub fn default_data_dir() -> Result<PathBuf> {

As suggested in the linked discussion, we could add support for an environment variable to override this algorithm. Bike-shedding on the name of that variable, I suggest SPIN_DATA_DIR.

In addition to adding this support, a bit of related clean-up would be nice:

• Replace TEST_PLUGINS_DIRECTORY with this new env var
• Maybe rename default_data_dir to...spin_data_dir (?); there isn't anything really "default" about it, especially with the removal of TEST_PLUGINS_DIRECTORY

Separately, should we do the same for "cache" (used by badger, OCI) and "config" (OCI registry auth) dirs?

cc @jandubois, @itowlson