Fgac changes

[asthamohta]

The PR includes generated changes as well, please just review the files in the following folders

1. Samples
2. src/database,session,session-pool
3. system-test
4. test

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[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/add-and-drop-new-database-role.js:26, tag spanner_add_and_drop_new_database
• samples/enable-fine-grained-access.js:29, tag spanner_enable_fine_grained_access
• samples/get-database-roles.js:26, tag spanner_list_database_roles
• samples/read-data-with-database-role.js:26, tag spanner_read_data_with_database_role

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