Add 'Any' type for JSON serializable types

[KendallWeihe]

Originated from this PR #130

We have needs for a general "any" type. The intent here is to create a solution which is compatible both in a pure rust development environment but also in a UniFFI binding project.

There is a question of: "should we place this in bindings/uniffi instead? because it seems to be a constraint specific to UniFFI"

The problem is on this line in web5.udl

interface VerifiableCredential {
  constructor(sequence<string> context, string id, sequence<string> type, string issuer, i64 issuance_date, i64? expiration_date, CredentialSubject credential_subject);

From @decentralgabe's comment here:

1. Pure native rust (with generics)
2. Bindings (with optional translation code to make the bindings nice)
3. Consumption of bindings
4. Idiomatic API surface in target language

I have had a stretch goal of limiting the burden on layer (2) to solely associated functions because leveraging the constructor feature in UDL means the binded language can do a traditional VerifiableCredential(...params). But in order for us to leverage the constructor feature, the crates/credentials/src/vc.rs would have to inline the Any type in it's code. And if Any lives in bindings/uniffi crate then that means our core crate now has a dependency on the UniFFI bindings crate. Which is no bueno.

All of this to say, bindings really turns us into a pretzel. This PR is intended to spur discovery, we may choose to decline merging this. We need to establish patterns which are repeatable.

[KendallWeihe]

So to be clear, the alternative could be to place this Any inside of the bindings/uniffi crate, and instead of this UDL code (added ... for readability, not actually valid code)

interface VerifiableCredential {
  constructor(...);

We would add this

namespace web5 {
  VerifiableCredential new_verifiable_credential(..., evidence: Any);

[KendallWeihe]

I have to write the full code to showcase the alternative. Doing so now. Moved this to a draft stage.

[decentralgabe]

nice - cool to prove this out