firestore support please

[u007]

Is your feature request related to a problem? Please describe.
migration support for firestore, probably via nodejs / typescript function or golang

Describe the solution you'd like

export const addUser = () : Promise<boolean> => {
  //todo...
  return true
}

Describe alternatives you've considered
possible can do it in golang as well

Additional context
non

[dhui]

I'm not familiar with Firestore. It'll need to support the following for migrations to make sense:

1. A query language
   • Namely some way to generically run different commands/functions/expressions.
   • Not sure if this is what @u007 was referring to when they said:

     probably via nodejs / typescript function or golang

2. A schema
   • Doesn't look like this is the case since it looks like a NoSQL document store
   • This isn't strictly necessary, but generally, migrate should only be used for schema migrations, not for populating/seeding DBs.

Feel free to add support for Firestore DB migrations if (1) is met. Add a note/caveat in the README if (2) is not met.
The db driver implementation will need to be in Go since that's what migrate is written in and will probably end up using: https://godoc.org/cloud.google.com/go/firestore