Extend OCM Plugins to support generic actions + define oci.repository.prepare action to assure existence of OCI repositories

[mandelsoft]

What this PR does / why we need it:

AWS ECR registries require the creation of an OCI repository before something can be uploaded.

To better support such special cases without flooding the common library with special cases
the plugin concept has been extended to support general typed actions.

The library now uses an action oco.repository.prepare to assure the existence of an OCI repository
before content is pushed.

Then an ecrplugin (under cmds/ecrplugin) is provided, which implements this special behaviour
for private ecr repositories.

Just copy the executable into your ~/.ocm/plugin folder.

Checkout its features with ocm describe plugin ecrplugin or ocm get plugins.

Unfortunately the ECR credentials cannot be used to access the AWS rest API required to create OCI repositories.
Therefore we need some kind of credential mapping. THis has been done based on plugin info and the spec of the action attributes.

As a result, the ecrplugin required appropriate additional AWS credentials which have bo be configured
in the .ocmconfig with the consumer type AWS

type: generic.config.ocm.software/v1
configurations:
  - type: credentials.config.ocm.software
    consumers:
      - identity:
          type: AWS
        credentials:
          - type: Credentials
            properties:
              awsAccessKeyID: "AKIAVWO6XXXXXXXXXXN6"
              awsSecretAccessKey: "XXXXXXXXXXXXXXXXXXXXXXXXX"

Which issue(s) this PR fixes:
Fixes #317

Special notes for your reviewer:

Release Notes: