Support for pre-services setup steps

[n1hility]

The services capability is an elegant way to automate the setup and tear down of a service, but there are use-cases where preparation tasks need to occur in a hook before services are started.

Possible examples include:

• Adjusting system parameters (e.g. file descriptor limits, socket buffer sizes, etc)
• Creating filesystem resources shared/used by containers
• Configuring ACLs
• Configuring network adapters and ip filters
• Installing packages

See https://github.com/actions/virtual-environments/issues/348 for an example use case of a workaround (although note, it will be solved by other means)

One form this could take would be something like:

runs-on: ubuntu-latest
setup-steps:
  - name: Stop Mysql
     shell: bash
     run: |
       sudo service mysql stop || true
services:
  # services
steps:
  # post-services steps