refactor: implement AI Client

[cwaldren-ld]

This implements the guts of the AI client, including fetching Model configuration, interpolation, and tests.

[cwaldren-ld]

There is some duplication here. This file is meant to contain classes that can be directly JSON deserialized, but this doesn't lend itself to the user-facing types. For instance, these fields have get; set; and I don't want that to be accessible to users. So, I deserialize these and then convert them into LdAiConfig.

I can't make these internal (deserialization seems to fail) and I can't use init; (we still support .net 4.6.2.)

I may be missing a more idiomatic way.

[kinyoklion]

You should be able to tell it to use a parameterized constructor JsonContructor which allows creating immutable types, but you would have to have a full parameterized constructor for the records (because no init).

[cwaldren-ld]

Updated to pass defaultValue.ToLdValue() directly into the underlying SDK. This recursively converts the existing LdAiConfig into an LdValue.

Note: if there are cycles in the Model dict, then bad things would happen (re: Ryan's comment.)