Allow config settings for API Gateway tags per stage

Permit specification of key/value pairs for API Gateway stage tags. The existing functionality for tagging Lambda functions is great, but it's also needed for API Gateway stages as well.

Start by locating the existing configuration and implementation used to tag Lambda functions, then trace where API Gateway stages are created or updated. Add equivalent key/value stage-tag configuration and verify that the configured tags are applied to stages without affecting Lambda tagging.

Written by the indexing model from the issue text.