feat(dbapi): manual commit sample

[IlyaFaer]

Adding a manual commit (!autocommit mode) sample.
The branch based on the branch from another PR, so it should be merged after #172 merged.

[snippet-bot]

Here is the summary of changes.

You added 2 region tags.

• samples/samples/autocommit.py:14, tag spanner_enable_autocommit_mode
• samples/samples/manual_commit.py:14, tag spanner_manual_commit

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