Add a 'summary' field in buildspec

[shahzebsiddiqui]

The summary field can can be used to describe a test which can be used as extended description. Currently the description field is limited to 80 char defined in https://github.com/buildtesters/buildtest/blob/devel/buildtest/schemas/definitions.schema.json

    "description": {
      "type": "string",
      "description": "The ``description`` field is used to document what the test is doing",
      "maxLength": 80
    },

The idea would be to include similar field without any maxLength. This would be applied to all schemas including script, compiler, and spack.

A typical test could be something like

buildspecs:
  summary_example:
    type: script
    description: Short description limited to 80 characters
    summary: |
      This is a long description of test that 
      can exceed 80 characters and be multiline 
   executor: generic.local.bash
   run: pwd

• Update schema https://github.com/buildtesters/buildtest/blob/devel/buildtest/schemas/definitions.schema.json and add property to other schemas such as script schema https://github.com/buildtesters/buildtest/blob/devel/buildtest/schemas/script.schema.json
• Add an example test in https://github.com/buildtesters/buildtest/tree/devel/buildtest/schemas/examples to ensure schema validates with summary field.
• Ensure test pass via regression test. Run via python $BUILDTEST_ROOT/buildtest/tools/unittest.py -c or buildtest unittests -c. See https://buildtest.readthedocs.io/en/devel/contributing/regression_testing.html
• Add example buildspec in https://github.com/buildtesters/buildtest/tree/devel/tutorials for documentation purposes.
• Document this field in https://github.com/buildtesters/buildtest/blob/devel/docs/buildspecs/buildspec_overview.rst