Support shorter ways to specify an instance

[enocom]

Right now a user must provide a full instance URI to launch the proxy. This is onerous, error-prone, and painful. The proxy should support some kind of shorthand to make it easier to launch the proxy.

One option would be to use something like Cloud SQL's instance connection name, e.g.,

alloydb-auth-proxy project:region:cluster:instance-name

Related to https://github.com/GoogleCloudPlatform/alloydb-auth-proxy/issues/26.

[kurtisvg]

Maybe tangential, but we should create a request to the UI / API teams to make whatever instance resource name we expose be easily discoverable/copypastable like it is Cloud SQL