Add free from "tags" field in the dataset + column model

[cccs-jc]

Is your feature request related to a problem? Please describe.
Superset can render dates using a data time pattern configured per column.

We would like to render other business types like IP, email etc. For example rendering a link to other web apps or formatting the data which is stored differently then how it is rendered. We are implementing a custom viz to do this. However we need a means to attach metadata information to the columns in the dataset model.

Describe the solution you'd like
We would like to be able to "tag" columns with additional information. Perhaps something similar to the "extra" field found on the dataset but on a column basis and also available on the client side.

Describe alternatives you've considered
At the moment we are using the type field of the column and setting it to IPv4, Domain, Email. However this field is probably not the best choice.

[junlincc]

@cccs-jc the request you have is similar to in-place adding column description on semantic layer?
we do have plan to implement it, but would love additional help from you :)