samples(spanner): add sample for Proto columns

[harshachinta]

This PR adds samples for Proto columns

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/src/main/java/com/example/spanner/AddProtoColumnSample.java:19, tag spanner_add_proto_type_columns
• samples/snippets/src/main/java/com/example/spanner/QueryWithProtoParameterSample.java:19, tag spanner_query_with_proto_types_parameter
• samples/snippets/src/main/java/com/example/spanner/UpdateProtoDataSample.java:19, tag spanner_update_data_with_proto_types
• samples/snippets/src/main/java/com/example/spanner/UpdateProtoDataSampleUsingDml.java:19, tag spanner_update_data_with_proto_types_with_dml

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