Reduces boilerplate when creating client instances

[juanpedromoreno]

This PR reduces some boilerplate when creating RPC client instances:

Previously to this change, in order to create new client instances, users have to do something like the following:

import cats.effect.IO
import freestyle.rpc.client._
import io.grpc.ManagedChannel //notice a io.grpc.import

val channelFor: ManagedChannelFor = ManagedChannelForAddress("localhost", 8080)

val channelConfigList: List[ManagedChannelConfig] = List(UsePlaintext(true))

val managedChannelInterpreter =
  new ManagedChannelInterpreter[IO](channelFor, channelConfigList)

val channel: ManagedChannel = managedChannelInterpreter.build(channelFor, channelConfigList)

val client: YourService.Client[IO] = YourService.client[IO](channel)

After this change, channelConfigList would be optional since it has a default value: _ = List(UsePlaintext(true)). Additionally, clients won't receive a io.grpc.ManagedChannel as first argument anymore, instead it'll receive the ManagedChannelFor object. Hence, this would be the equivalent version of the code above:

val channelFor: ManagedChannelFor = ManagedChannelForAddress("localhost", 8080)
val client: YourService.Client[IO] = YourService.client[IO](channelFor)

Optionally, as we've mentioned, you could still pass a List[ManagedChannelConfig] as a second argument.

[codecov-io]

Codecov Report

Merging #97 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #97   +/-   ##
=======================================
  Coverage   83.21%   83.21%           
=======================================
  Files          12       12           
  Lines         137      137           
=======================================
  Hits          114      114           
  Misses         23       23

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c65515c...5c14ffc. Read the comment docs.

[fedefernandez]

Looks great!