Support `apicurio` as `catalog` for specifications

[jfallows]

Describe the desired outcome from the user's perspective
As devops, I want to be able to store my OpenAPI and AsyncAPI specification in apicurio instead of on the filesystem relative to zilla.yaml.

Acceptance criteria

• openapi specification can be retrieved from apicurio via catalog handler
• asyncapi specification can be retrieved from apicurio via catalog handler

Tasks

• Add Apicurio catalog #827
• Add Apicurio catalog documentation zilla-docs#198

Additional context
See apicurio rest api v3