feat: add support for JSON data type

[olavloite]

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[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• samples/json-add-column.js:18, tag spanner_add_json_column
• samples/json-query-parameter.js:18, tag spanner_query_with_json_parameter
• samples/json-update-data.js:18, tag spanner_update_data_with_json_column

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[olavloite]

(Closing this in favor of #1368. This PR was only intended to test whether the same UNAUTHENTICATED error would happen when the PR was created directly in the repo, as opposed to in a fork. As the same error happens in this case as well, there's no need for this PR.)