Added variables supporting separate additional tags for writer and reader

[bergbrains]

Adds ability to assign additional tags to writer and reader

• Additional tags are merged with module.this.tags
• Separate variables for reader and writers

To allow observability at the db-role level

• In order to provide a way to query for the reader and writer in metrics/events/logs

[bergbrains]

The auto-format error is really a perms issue.

I added cloudposse bot as a contributing writer, after these initial errors.

[nitrocode]

Let's make the names consistent with this PR https://github.com/cloudposse/terraform-aws-alb/pull/122/files

[nitrocode]

@bergbrains please run make pr/auto-format locally to fix the auto format errors

[bergbrains]

Maybe i'll get to this again someday.