Allow generating context as an implicit parameter

[iRevive]

Motivation

With the recently released #669, there is a new scenario where the context can be extended, and it can be passed implicitly to inner functions.

Let's consider the following example:

class Ctx(...) 
class TracingMetadata(tracingCtx: Ctx, metadata: io.grpc.Metadata)

class Service extends ServiceFs2GrpcTrailers[IO, TracingMetadata] {

  def endpoint(request: Request, ctx: TracingMetadata): IO[Response] = {
    given Ctx = ctx.tracing
    for {
      _ <- log("handling request")
      r <- logic(request)
    } yield r
  }
  
  def logic(r: Request)(using Ctx): IO[Response] = ???
  def log(str: String)(using Ctx): IO[Unit] = ???
  
}

The usage is a little noisy and repetitive. With the new flag, we can put ctx into an implicit position in the generated service:

given (using m: Meta): Ctx = m.c

class Service extends ServiceFs2GrpcTrailers[IO, TracingMetadata] {

  def endpoint(request: Request)(using ctx: TracingMetadata): IO[Response] = 
    for {
      _ <- log("handling request")
      r <- logic(request)
    } yield r
  
  def logic(r: Request)(using Ctx): IO[Response] = ???
  def log(str: String)(using Ctx): IO[Unit] = ???
  
}

[ahjohannessen]

I think the idea of opt-in for generation of using/implicits is nice wrt. ergonimics.

[iRevive]

@ahjohannessen this one is rebased and squashed.

[ahjohannessen]

Great work! 👍

[iRevive]

@ahjohannessen thanks!

What's your plan for the release? Should we have the RC first?

[ahjohannessen]

@iRevive I have released v3.0.0. Do not think a RC is necessary. The work you have done is opt-in, so it is not that disruptive.

[iRevive]

@iRevive I have released v3.0.0. Do not think a RC is necessary.

Perfect, thank you!