[spanner] Add TransactionManager

[danielnorberg]

Implements the TransactionManager API as described in issue #375. This allows manual control over transaction execution while maintaining session reuse across retries, which helps retain lock priority and improves commit success rates for ABORTED transactions.

Key changes:

• Add TransactionManager struct that holds a session across multiple transaction attempts
• Add Client::transaction_manager() method to create a TransactionManager
• Add Debug implementation for BeginError to support unwrap() in user code
• Add comprehensive integration tests for TransactionManager functionality

The API enables users to handle transaction retry logic manually while benefiting from session reuse:

let mut tm = client.transaction_manager().await?;
let retry = &mut TransactionRetry::new();
loop {
    let tx = tm.begin_read_write_transaction().await?;
    let result = run_in_transaction(tx).await;
    match tx.end(result, None).await {
        Ok((commit_result, success)) => return Ok(success),
        Err(err) => retry.next(err).await?
    }
}

Fixes #375

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