#6 Add BulkPairings.export_games

[nstillman-te]

Part of #6.

Add client.bulk_pairings.export_games for GET /api/bulk-pairing/{id}/games: export games of a bulk pairing as PGN or NDJSON stream. Supports moves, tags, clocks, evals, accuracy, opening, division, literate, pgnInJson. Uses existing Game model for NDJSON; requests-mock test (endpoint requires auth).

Checklist when adding a new endpoint

• Added new endpoint to the README.rst
• Ensured that my endpoint name does not repeat the name of the client (export_games, not bulk_pairing_export_games)
• Typed the returned JSON using TypedDicts in berserk/types/ (NDJSON response uses existing Game model; no new types)
• Written tests for GET endpoints not requiring authentification (endpoint requires auth; added requests-mock test for path and params)
• Added the endpoint to CHANGELOG.rst in the To be released section