Enable starting only a subset of components

[kate-goldenring]

As a feature request, the shim should be able to execute only a subset of components. This will likely pull in changes from the Spin CLI work around this: fermyon/spin#2820

[Mossaka]

Probably want a new Spin Execution Config variable like SPIN_COMPONENTS that takes a list of components. E.g.

env:
  - name: SPIN_COMPONENTS
    value: "hello-rust,hello-go"

[kate-goldenring]

Thats exactly what i have in mind. I have a working implementation with SPIN_COMPONENTS_TO_RETAIN as the env. I'll put up a PR once the Spin change merges