feat: output VRS digest with post mapped HGVS strings

[bencap]

This pull request adds support for extracting and exporting the VRS digest from post-mapped variant objects. The main changes involve updating the CSV export logic to include the digest, implementing the extraction function, and ensuring robust test coverage.

VRS Digest Extraction and Export:

• Added a new function get_digest_from_post_mapped in mavedb.lib.variants to extract the digest field from post-mapped VRS objects.
• Updated get_score_set_variants_as_csv and variant_to_csv_row in mavedb.lib.score_sets to include the post_mapped_vrs_digest column in exports when post-mapped HGVS columns are included. [1] [2]

Testing Improvements:

• Added a suite of tests for get_digest_from_post_mapped to verify correct extraction behavior for various input scenarios in tests/lib/test_variants.py.

Codebase Cleanup and Imports:

• Cleaned up and reordered imports for clarity and consistency in mavedb.lib.score_sets and mavedb.lib.variants. [1] [2] [3]
• Updated import statements in tests/lib/test_variants.py to include the new function and improve readability. [1] [2]