Registry tool: support --- separator between documents in Registry YAML files

[timburks]

Kubernetes YAML allows multiple documents to be concatenated in a single file, separated by a line with three dashes.

https://stackoverflow.com/questions/50788277/why-3-dashes-hyphen-in-yaml-file

This might be useful for Registry YAML. For example, we might group all the patch versions of a Kubernetes minor version into a single file:

apiVersion: apigeeregistry/v1
kind: Version
metadata:
  parent: apis/kubernetes
  name: v1.26.3
data:
  displayName: v1.26.3
  specs:
  - metadata:
      name: openapi
    data:
      filename: swagger.json
      mimeType: application/x.openapi+gzip;version=2.0
      sourceURI: https://raw.githubusercontent.com/kubernetes/kubernetes/v1.26.3/api/openapi-spec/swagger.json
---
apiVersion: apigeeregistry/v1
kind: Version
metadata:
  parent: apis/kubernetes
  name: v1.26.2
data:
  displayName: v1.26.2
  specs:
  - metadata:
      name: openapi
    data:
      filename: swagger.json
      mimeType: application/x.openapi+gzip;version=2.0
      sourceURI: https://raw.githubusercontent.com/kubernetes/kubernetes/v1.26.2/api/openapi-spec/swagger.json
---
apiVersion: apigeeregistry/v1
kind: Version
metadata:
  parent: apis/kubernetes
  name: v1.26.1
data:
  displayName: v1.26.1
  specs:
  - metadata:
      name: openapi
    data:
      filename: swagger.json
      mimeType: application/x.openapi+gzip;version=2.0
      sourceURI: https://raw.githubusercontent.com/kubernetes/kubernetes/v1.26.1/api/openapi-spec/swagger.json
---
apiVersion: apigeeregistry/v1
kind: Version
metadata:
  parent: apis/kubernetes
  name: v1.26.0
data:
  displayName: v1.26.0
  specs:
  - metadata:
      name: openapi
    data:
      filename: swagger.json
      mimeType: application/x.openapi+gzip;version=2.0
      sourceURI: https://raw.githubusercontent.com/kubernetes/kubernetes/v1.26.0/api/openapi-spec/swagger.json

[timburks]

We already support this, which is produced by registry get and read by registry apply:

apiVersion: apigeeregistry/v1
items:
  - apiVersion: apigeeregistry/v1
    kind: Version
    metadata:
      name: v1.10.0
      parent: apis/kubernetes
      labels:
        updated: "2023-03-19"
    data:
      displayName: v1.10.0
      state: production
  - apiVersion: apigeeregistry/v1
    kind: Version
    metadata:
      name: v1.10.13
      parent: apis/kubernetes
      labels:
        updated: "2023-03-19"
    data:
      displayName: v1.10.13
      state: production
  - apiVersion: apigeeregistry/v1
    kind: Version
    metadata:
      name: v1.11.0
      parent: apis/kubernetes
      labels:
        updated: "2023-03-19"
    data:
      displayName: v1.11.0
      state: production

[theganyo]

Multiple docs are easily supported. We used multi-doc CRD format for our standard configuration in envoy adapter, see: https://github.com/apigee/apigee-remote-service-envoy/blob/main/server/config.go#L172.