Add ability to use standard generators in Ax generation strategies

[thopkins32]

The standard adopted by other optimization libraries such as Xopt, optimas, etc. for generators can be integrated into Blop via Ax's External Generation Nodes which would sit as part of a custom generation strategy.

We should implement the generator standard API and put it in an External Generation Node. That way, if someone wants to use an Xopt model within Blop, they can!

There is also an idea of making the generator completely separate from the Bluesky plan, but I'm not sure how this will work in practice just yet. Right now, they are tightly coupled (see blop.ax.Agent.acquire). If this is possible, someone may be able to use Bluesky + any gest-api supporting package directly.

[thopkins32]

Waiting on response from campa-consortium/gest-api#53 before continuing with this.