Reusable pipeline steps

[Hofer-Julian]

One common problem with Rattler-Build recipes is, that you have a sequence of build steps that are duplicated across multiple recipes. GitHub actions solve this problem with reusable workflows: https://docs.github.com/en/actions/how-tos/reuse-automations/reuse-workflows

This issue is about adding reusable pipeline steps to recipe.yaml inspired by GitHub Action's reusable workflows.

From now on we assume that #2646 is merged. This pull request adds experimental build.steps as a structured alternative to build.script. Steps are currently anonymous and local to a single recipe. When multiple recipes follow the same build procedure, each of them has to repeat the whole sequence.

One way of describing this could be:

• steps get a name
• a standalone file defines a sequence of named steps together with the inputs it accepts
• recipe.yaml references that file and sets the inputs

# cmake-steps.yaml
inputs:
  extra_args:
    default: []

steps:
  - name: configure
    run: cmake $SRC_DIR -B build ${{ inputs.extra_args | join(" ") }}
  - name: build
    run: cmake --build build
  - name: install
    run: cmake --install build --prefix $PREFIX

# recipe.yaml
build:
  steps:
    - uses: cmake-steps
      with:
        extra_args: ["-DBUILD_TESTING=ON"]