Support for pluggable Template sources

[wbreza]

Today the azd template list command displays a static list of available templates curated internally by the AZD team and controlled via a source controlled JSON file.

Requirements

1. Ability to reference 1 or many external template sources
2. Template sources support JSON or YAML files

Template Source Types

1. FileSystemTemplateSource - References a local file path
2. UriTemplateSource - Referenced an HTTP/HTTPS addressable URL
3. CustomTemplateSource - Custom implementation that returns list of externally managed templates (ADE)

Tasks

Beta Give feedback

1. Design template source schema
2. Implement FileSystemTemplateSource
3. Implement UriTemplateSource
4. Update azd template list command
5. Design configuration schema for template sources
6. Implement DevCenterTemplateSource