Use string instead of int64 for projectID in query APIs

This change is an important precursor to the cloud connector changes. The cloud connector will send projectIDs from self-managed instances as a projectID+instanceID combination to ensure uniqueness.

We currently parse all request path strings to get the int64 value of projectID to pass the int64 around before converting back to a string in SQL which is unnecessary - we can just use the string value without converting to/from int64.

closes Alter query API code to use string for ProjectI... (#2762 - closed) • Mat Appelman • 17.1