Expand/Improve Functions ARM APIs

[mathewc]

Expand the set of Functions ARM APIs to cover scenarios that aren't possible currently via ARM. Another goal is to support the migration of the Functions portal to ARM apis solely, rather than accessing Kudu/Runtime APIs directly.

Some operations are already supported by existing ARM APIs even though the portal isn't using them currently. The new APIs needed include:

• host key apis (/host/keys), read/write/delete
• function key apis (/functions//keys), read/write/delete

This list isn't exhaustive.