feat: XML/JSON results more human-readable

[jkowalleck]

expected result:

• add an parameter to the serializes, that controls indention.
  default to "OFF" -- no indention and such
• if indention exists, then beautify JSON/XML
• one property declaration per line
• nested structures are indented

--> in addition, the test data - SNAPSHOTS - should

• be marked as "binary" and compared byte-wise, not deep-content-wise.
  these files are designed to be human-readable, not human-maintainable.
• be generated with an indention of 2 spaces for better human readability and showcasing this feature.

enables CycloneDX/cyclonedx-python#424