Scala API for Scio Transforms

[anish749]

We have some awesome transforms in Scio's transforms package. When using them in Scala pipelines, it can be less idiomatic. It would be nice to have first class support for this.

I was using RateLimiterDoFn and DoFnWithResource and I was hoping to have something like these on an SCollection[T]

def mapWithResource[R, U: Coder](resource: => R, resourceType: ResourceType)(fn: (R, T) => U): SCollection[U]

or

def rateLimitPerWorker(recordsPerSecond: Double): SCollection[T]

I can send a PR with some of the stuff if it fits in here.

[regadas]

@zwycl will take a stab at this.

[l0kr]

Since https://github.com/spotify/scio/pull/3389 is merged, should this task be closed?