Init Resource Handlers for AWS::CodeArtifact::Repository

[jonjara]

Basic implementation of Create/Read/Delete handlers.

Description of changes: Template code from cfn-init and implemented Create/Delete/Read handlers.

Testing Did cfn-submit on my own account, created a stack with a repository/domain template and deleted it.

Some TODOs:

• update/list handlers
• Add upstream, external connection, and policy handling for create/update handlers and to the resource schema

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.