Add method to dump repo dbs to JSON files

repo_management/operations.py:
Add `dump_db_to_json_files()` which allows for a repository database
file to be read and all of its members to be serialized as JSON and
written to files based upon the packages respective pkgbases.
For the JSON serialization orjson is used, as it provides the highest
speeds available and seems the most correct implementation.

tests/test_operations.py:
Add a fixture to create and destroy a temporary directory (used to write
JSON files to).
Add tests for `dump_db_to_json_files()`.
2 jobs for issues/2 in 50 seconds (queued for 4 seconds)
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passed #16837
coverage

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passed #16836
linter

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