Adds default custom scalar of interface{}

[qhenkart]

Given that the Map default custom scalar is automatically provided of map[string]interface{}, it seemed strange to me that interface{} itself has no scalar. These seems like a common (albeit not optimal) enough design pattern in Go that it should be included. At first I thought it might push people not to strictly type their schema, but since Map is supported, I don't see any reason why this shouldn't be.

I didn't add and link an issue because of the simplicity of this PR

I have:

• [] Added tests covering the bug / feature (see testing)
  I can add tests if they are deemed necessary, but the other custom scalars don't have any, and this particular scalar is much simpler
• Updated any relevant documentation (see docs)

[vektah]

I'm on the fence about this, Map and Time are universal, Interface is leaking goisms.

OTOH I can see it might be useful.

Any thoughts @mathewbyrne?

[mathewbyrne]

I don't love the name Interface in GraphQL since it's a Go-ism, we're thinking maybe Any instead?

[qhenkart]

@vektah @mathewbyrne I originally went with Untyped (that's what I ended up calling my custom scalar anyway). I wasn't sure what direction you guys would would want to take so I went with Interface for the PR. Any flows really nicely when writing out the schema, and is actually more of a noun than untyped which is more of an adjective. I will switch it to Any.

[vektah]

Awesome, thanks for the PR.

[JulienBreux]

👏 @qhenkart for work and 🙏 @vektah for merge!