Add Google Sheets state persister

[prvn0x]

Closes #213

Adds a state persister backed by a Google Sheets worksheet, so applications can
persist and reload state in a store that is human-readable and shareable without
running a database. This builds on the earlier work in #670 by @goutamk09 and
addresses the review feedback left there.

Changes

• burr/integrations/persisters/b_google_sheets.py — GoogleSheetsBasePersister,
  implementing BaseStatePersister. State is stored one row per step, keyed by
  (partition_key, app_id, sequence_id, position), with a header row written by an
  idempotent initialize().
• pyproject.toml — a google-sheets extra so gspread stays an optional
  dependency, wired into the tests extra.
• docs/reference/persister.rst — table entry and autoclass reference.
• tests/integrations/persisters/test_b_google_sheets.py — 16 unit tests.

Specific points raised in the review of #670:

• load() scans to the last matching row rather than the first, so state is
  still readable after repeated saves to the same app.
• All reads go through a single _data_rows() helper, so the header row is skipped
  consistently rather than in some methods but not others.
• save() and load() match the BaseStatePersister signatures, including
  sequence_id.
• No spreadsheet IDs or credentials are committed.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.