Add "tags" and "meta" attributes to Exposure schema

[stkbailey]

Describe the feature

I would like to be able to tag and add metadata to exposures, so that dbt can serve as a system of record for major data-related initiatives. The current options are solid, but we have additional metadata we would like to add -- primarily so that we can use the exposure data in downstream systems.

Describe alternatives you've considered

We can use some of the existing data fields but additional flexibility would be nice.

Additional context

This feature is mentioned by a few commenters in issue 2835 but it does not look like action has been taken.

Who will this benefit?

This will be useful for people using exposures and will also bring the object into closer alignment with other resources.

Are you interested in contributing this feature?

Yes, I will link the PR.

[jtcohen6]

@stkbailey I so agree. Adding tags and meta feels totally uncontroversial to me. There are lots of other directions I could see exposures heading in; this change wouldn't close any doors for us, and it would definitely represent an improvement over status quo.

Thanks for splitting this out from #2835, where it was getting lost in some very interesting discussion :)