Support wasi-config

[kate-goldenring]

WASI is moving wasi-config to phase 3 soon. We should consider moving from the spin variables interface to WASI config.

The main diff is the need to add support for get-all

Spin 3.0 may be a good opportunity to make this switch

[kate-goldenring]

I did a quick check on implementation feasibility of get-all for our current providers:

• For redis, taking a quick look, we should be able to keys and then mget on that set of keys: https://docs.rs/redis/latest/redis/trait.Commands.html
• For Azure, they provide a list_secrets command that should do the trick: https://docs.rs/redis/latest/redis/trait.Commands.html
• For env var, we just return the kv pairs for env vars with the expected SPIN_VARIABLES prefix

[lann]

get-all isn't a huge problem for Spin because all variables must be declared in the manifest, so its really just a get-many, which can itself be implemented in terms of plain get if needed.

[kate-goldenring]

@lann your point makes me think that the WASI config interface may want to rename that function. It defines get-all as getting the set of KV pairs:

    /// Gets a list of configuration key-value pairs of type `string`.
    /// 
    /// If an error occurs, an `Err(error)` is returned.
    get-all: func() -> result<list<tuple<string, string>>, error>;