feat: Add automatic PDML fallback for mutation limit errors

[aakashanandg]

Description:
This change introduces an opt-in, automatic retry mechanism for DML statement that fails due to Spanner's transaction mutation limit.

When a DML statement fails with a TransactionMutationLimitExceededError, this logic will automatically retry the statement within a new Partitioned DML (PDML) transaction.

Key Features:

• The fallback is transparent and enabled by passing a fallback_to_pdml isolation option to a standard transaction block.
• If the fallback is disabled or unsafe, a specific TransactionMutationLimitExceededError is raised.

Usage:

Singer.transaction(isolation: fallback_to_pdml) do
  # If this fails with a mutation limit error, it will be retried with PDML.
  Singer.where("last_name LIKE ?", "A%").update_all(status: "Archived")
end

[olavloite]

LGTM, with some minor nits/suggestions on the tests and sample.