Check for Architecture during Registry Build

[maysunfaisal]

Please specify the area for this PR
index/generator

What does does this PR do / why we need it:
Checks the architecture list during the registry build and outputs a warning to stdout if the architecture list is empty. Its best practice to mention the architecture when authoring devfiles but its not mandatory. Check devfile/api#244 (comment) for more info

Which issue(s) this PR fixes:

Addresses part of comment devfile/api#244 (comment)

PR acceptance criteria:

• Test (WIP)

• Documentation (WIP)

How to test changes / Special notes to the reviewer:
build and run

 ./index-generator ../../tests/registry /tmp/index.json
 
the java-maven devfile has no architecture(s) mentioned
the java-quarkus devfile has no architecture(s) mentioned
the nodejs-basic devfile has no architecture(s) mentioned
the code-with-quarkus devfile has no architecture(s) mentioned
the java-springboot-basic devfile has no architecture(s) mentioned
the python-basic devfile has no architecture(s) mentioned

Index Example with Arch:

  {
    "name": "nodejs",
    "version": "1.0.0",
    "displayName": "NodeJS Runtime",
    "description": "Stack with NodeJS 12",
    "type": "stack",
    "tags": [
      "NodeJS",
      "Express",
      "ubi8"
    ],
    "architectures": [
      "amd64"
    ],
    "projectType": "nodejs",
    "language": "nodejs",
    "links": {
      "self": "devfile-catalog/nodejs:latest"
    },
    "resources": [
      "devfile.yaml"
    ],
    "starterProjects": [
      "nodejs-starter"
    ]
  },

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnmcollier, maysunfaisal

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