feat: add publisher gateway client

[lengau]

This adds a very basic publisher gateway client that meets the needs of charmcraft's new 'create-track' command.

The client cannot login yet and still depends on the old requests-based clients for many features.

Potentially resolves #239

CRAFT-3424

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[dariuszd21]

Isn't it better to use MarshableModel instead of TypedDict ?

[lengau]

We're trying to move away from generating and transforming the models all the time. This allows us to return the JSON dictionary returned by the store but still provide users with helpful details and linting during development.

At runtime, these are a no-op, but during development they offer both this:

and this:

[dariuszd21]

I won't block you from delivering that, but for me it looks terrible and is really bug-prone - no validation at runtime unless you explicitly do it by yourself. No control over values returned from the store and

At least we should use a dataclass-like regular syntax to make it somehow readable.
https://docs.python.org/3.12/library/typing.html#typing.TypedDict

I think at some point, we should also enforce using TypeAdapters for validating all the data that gets in and out:
https://docs.pydantic.dev/latest/concepts/type_adapter/#__tabbed_1_2

[lengau]

This was my understanding of what @sergiusens intended - for us to be a pretty slim wrapper around the actual API. I think his intention was for us to pass the data structures as-is so that we don't need to update craft-store to be able to use additional properties that get added on the store side (either in the input or in the output).

[dariuszd21]

Looks good for me!