Support `@extschema:name@`

[jkatz]

Describe the problem

Extensions that have dependencies on other extensions need awareness of what schema the dependent extension is installed into. However, it may not be possible to install dependent extensions into other schemas, particularly when an extension is relocatable.

Describe the proposal

postgres/postgres@72a5b1f (targeted PostgreSQL 16) added a new substitution, @extschema:name@, that allows inserting the schema name of a required extension to simplify cross-extension references.

Describe alternatives

A user set up their extensions so that they are in the appropriate reference schema.