1ES pipeline templates

[NikolaMilosavljevic]

Splits the existing CI pipeline into two separate pipelines:

• azure-pipelines-PR.yml: for public builds - This is the same as the original content except that conditional logic has been removed that was meant to support internal builds.
• azure-pipelines.yml: for internal builds - Uses the 1ES pipeline template pattern

Successful validation builds:

• internal: https://dev.azure.com/dnceng/internal/_build/results?buildId=2414346&view=results
• public: https://dev.azure.com/dnceng-public/public/_build/results?buildId=617819&view=results

[NikolaMilosavljevic]

Validation build was successful: https://dev.azure.com/dnceng/internal/_build/results?buildId=2415153&view=results

[MichaelSimons]

@NikolaMilosavljevic - friendly reminder to backport for all servicing branches. If we will have no more builds in 7.0, that could be skipped.