feat: support optional tag binding

[bharathkkb]

fixes #884

[g-awmalik]

Thanks for taking care of this. My comments center around the lack of usage of google_tags_tag_key in the module? AFAIK, the key and the value are paired together for the binding and we should give the user the ability to create both. What do you think?

[bharathkkb]

My comments center around the lack of usage of google_tags_tag_key in the module? AFAIK, the key and the value are paired together for the binding and we should give the user the ability to create both. What do you think?

My reasoning for this was the common use case seems to be that keys (and values) are defined at a higher level (org/folder) and only binding is performed at the project during creation time. There could be usecases where maybe project scoped tags are created but I am thinking we should recommend use the tags module for that.

This would be similar to how we only allow attaching to a shared VPC via PF while requiring network module for full create functionality.

[apeabody]

A few minor suggestions - saw the existing comment one why this doesn't need to be a breaking change.