Add nightly feed publishing

[ReubenBond]

This adds a CI pipeline which publishes nightly builds of Orleans from the main branch.

• We cannot specify parameters or variables in YAML schedules, therefore we need something else.
• To avoid duplicating the main pipeline yaml, I moved the guts of build.yaml into a template, templates/build.yaml and reference that from the root build.yaml.
• Since templates cannot have schedules or triggers, those go into the base build.yaml as well as the template, and they're passed through explicitly as template parameters.
• nightly-main.yaml contains the nightly build publishing pipeline and it references templates/build.yaml, setting a publish_nightly parameter.
• Since you apparently cannot specify variables in templates which also have jobs, the variables go into templates/vars.yaml and we reference them from nightly-main.yaml and build.yaml

If I am wrong about the above rationale, please let me know - I am not an AzDO expert

Microsoft Reviewers: Open in CodeFlow

[ReubenBond]

@benjaminpetit PTAL