Enhancement: Disambiguate Feed Types

[jbristowe]

Is your feature request related to a problem? Please describe.
Feeds are ONLY differentiated by their associated FeedType. Furthermore, specific feeds -- such as ones for Docker -- are serialised incorrectly when used by the Terraform Provider for Octopus Deploy. This results in invalid plans to be generated through terraform plan. See Feed creation always has planned changes (#142) for more information concerning this behaviour.

Describe the solution you'd like
Feeds should be defined by their type (i.e. Docker container registry).

Additional context
The following types are to be implemented and support for feeds:

• AwsElasticContainerRegistry
• DockerContainerRegistry
• GitHubRepositoryFeed
• HelmFeed
• MavenFeed
• NuGetFeed