Update license scripts to support YAML frontmatter changes

[sarthak-shubham]

Fixes #3138

What

Updated report_license_rules.py and buildrules.py in etc/scripts/licenses/
to work correctly with the YAML frontmatter format introduced in #3100, which
added new fields (is_builtin, is_generic, is_continuous, length) to the
License and Rule models.

report_license_rules.py

• Added extrasaction='ignore' to the csv.DictWriter calls so the script no
  longer crashes when the models gain fields not present in the CSV column
  headers — also avoids needing to update this script every time a new model
  field is added going forward.
• Added encoding="utf-8" to the open() call in write_data_to_csv(), since
  the default Windows encoding (cp1252) can't handle non-ASCII characters
  (e.g. in author names) present in the license database.
• Added progress logging via click.echo() (start, periodic progress every
  5,000 rules, completion message) since exporting 35,000+ rules takes several
  minutes with no prior feedback that the script is working.
• Switched to streaming with enumerate() instead of loading all rules into a
  list upfront, to support the progress logging without changing the output.

buildrules.py

• Added a click.echo() message before the duplicate-check step
  (all_rule_by_tokens()). This step tokenizes all 35,000+ existing rules to
  check the new rule for duplicates, which takes roughly 10 minutes on a full
  run — expected given the dataset size, but silent without the log.

Testing

Ran both scripts manually against the current license/rule database —
report_license_rules.py completes end-to-end; buildrules.py correctly parses
an example rule file and generates a new .RULE file (deleted afterward to
avoid committing test artifacts).

I wasn't able to find existing automated tests covering these two scripts —
they appear to be maintainer-facing utility scripts outside the main src/
detection engine and its test suite. Is there a preferred location/pattern
for tests on scripts like these, or should I add something under tests/
following a different structure?

Note

Saw #3150 referenced in the issue as possibly covering part of this — happy
to adjust if there's overlap.

Also included

Added an AUTHORS.rst entry and a CHANGELOG.rst note per CONTRIBUTING.rst's
PR guidelines.