Secret Store

[joeshaw]

This introduces types and hostcalls for the Secret Store, and implements
them in Viceroy, along with configuration to instantiate them.

In Compute@Edge, a Secret Store is an encrypted, read-only key-value
store for sensitive data. In Viceroy, however, it is a simple
unencrypted in-memory map defined in the fastly.toml file in a
manner similar to Object Stores.

At a high level, a Wasm application using the APIs would:

1. Open a secret store by name
2. Get a secret from the store by name
3. Decrypt the secret by calling its plaintext method.

In Viceroy, Secret Stores are configured in the same way Object Stores
are:

[local_server]
  [local_server.secret_store]
    store_one = [{key = "first", data = "This is some secret data"}, {key = "second", path = "/path/to/secret.json"}]

    [[local_server.secret_store.store_two]]
      key = "first"
      data = "This is also some secret data"

    [[local_server.secret_store.store_two]]
      key = "second"
      path = "/path/to/other/secret.json"

[mgattozzi]

Hey @joeshaw I took a look and this is amazingly well done. I do see it's trying to merge it into the acf/finish-streaming-bodies branch right now so we should probably hold off until that's merged and then set this PR to merge into main instead after that's in, otherwise I think these changes have solid comprehensive test coverage and they're passing so I think we should be okay.

[joeshaw]

@mgattozzi Sounds good! Yeah, I wasn't planning this land this until the other branch has landed. Once that branch is landed, this PR should automatically re-target main.

[awilliams-fastly]

Adding a reminder for us to update https://developer.fastly.com/reference/compute/fastly-toml/#local-server

[joeshaw]

@mgattozzi With #203 having been merged and the 0.9.1 Rust SDK having been tagged, this should be ready to merge.

[joeshaw]

Rebased on main, which switched to Wasmtime 4.0.0 and required some tweaks (#217)

[mgattozzi]

@joeshaw thanks for rebasing things and getting it set to merge into main. I think this is ready to go!