feat: validate CycloneDX documents

[ptdropper]

Need a way to validate that my SBOM output file correctly meets the specification based upon a specific specification version.

[jkowalleck]

requirements:

• have functional tests for all supported CDX versions/formats
  based on https://github.com/CycloneDX/specification/tree/master/tools/src/test/resources
  (loader script can be taken from https://github.com/CycloneDX/cyclonedx-javascript-library/tree/main/tests/_data/schemaTestData
• API:
  • validator for atleast all currently implemented schema versions
  • input: version enum
  • output: None or an error object
  • throws: on unexpected behavipur or problems that prevent validation
  • does not throw: validation error --> these are to be returned
• use the existing schema resoources of https://github.com/CycloneDX/cyclonedx-python-lib/tree/main/cyclonedx/schema
  maybe update them based on latest data
• DOCS
  • add validation to the existing examples
  • maybe mention it in the README/capabilities/docs

[jkowalleck]

reminder: we have schema validation already implemented in the tests.
the used libs seam to work well, lets use them for the public API implementation.

reminder: we already ship the schema files.
lets use them for validation.

[jkowalleck]

this feature should be optional, so dependency bloat is minimized.
Therefore, lets have the dependencies as multiple install-extras: "validation", "xml-validation", "json-validation"

this shall be documented in code and in other places.

[jkowalleck]

some features require py38.
so py3.7 needs to drop -- #436

OOOOOOR

since the deps are optional anyways, they can be marked as py-version-dependent ;-)