Support shorter ways to specify an instance

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.

Start at the command-line entry point for alloydb-auth-proxy and read related issue #26 for existing context on instance specification. Clarify the shorthand syntax, validation, and compatibility expectations before implementation; the work is done when the proxy reliably launches from the agreed shorthand as well as a full instance URI.

Written by the indexing model from the issue text.