Finish the Policy Trait and implement a basic policy

[esoterra]

1. Wire policy through the server so that it is used to accept/reject publishes
2. Implement a basic policy that uses an allow list of keys/package names
3. Allow the basic policy to be deserialized from a config file

[peterhuene]

#121 implements configuring a server for rejecting any content that isn't WebAssembly (likely one we'd use for the dogfooding implementation).

Record-based policies (e.g. one where it would validate a public key id is accepted for the package name) are forthcoming.

[peterhuene]

#130 implements a record policy trait for rejecting published records by policy; a simple "authorized key" policy was also implement as part of it.

[peterhuene]

What's missing: ability to configure policies for warg-server CLI; currently policies can only be enabled via the API.