pipelines: Major improvements

[bwateratmsft]

Been working on some major improvements to our pipelines in #2174.

Some key things I want to address:

• Removal of obsolete things (code, parameters, configs)
• Using Git tags branches to version the pipelines
• Auto-discovery of VSIXs, their platforms, and associated proposed APIs, to enable automatically creating publish commands
• Batch signing all discovered VSIXs in parallel instead of individually
• Auto-discovery of tarballs for publishing
• Shared variables in shared YAML (instead of in AzDO variable groups)
• Less duplicate code in the repos, more sharing here
• Easier usage of test service connection for BAMI tenants
• Parameterized signing--to make the pipelines usable outside of MicroBuild/DevDiv

Additional possible goals:

• Standardizing on the .config folder for the pipelines, Guardian suppressions files, etc.

[bwateratmsft]

This should also solve #2160