[improvement] Use new picocli lib for CLI

[ferozco]

Before this PR

Previously, conjure-java failed on unknown command line arguments and had very poor feedback when used as a CLI.

After this PR

We have migrated to use picocli which gives us a more declarative API for defining command line args, rich usage docs and support for unknown command line options.

ex:

Usage: conjure-java generate [-hV] [--jersey] [--jerseyBinaryAsResponse] [--objects]
                             [--requireNotNullAuthAndBodyParams] [--retrofit] [--retrofitCompletableFutures]
                             [--retrofitListenableFutures] <input> <output>
Generate Java bindings for a Conjure API
      <input>      Path to the input IR file
      <output>     Output directory for generated source
      --jersey     Generate jax-rs annotated interfaces for client or server-usage
      --jerseyBinaryAsResponse
                   Generate jersey interfaces which return Response instead of StreamingOutput
      --objects    Generate POJOs for Conjure type definitions
      --requireNotNullAuthAndBodyParams
                   Generate @NotNull annotations for AuthHeaders and request body params
      --retrofit   Generate retrofit interfaces for streaming/async clients
      --retrofitCompletableFutures
                   Generate retrofit services which return Java8 CompletableFuture instead of OkHttp Call
      --retrofitListenableFutures
                   Generate retrofit services which return Guava ListenableFuture instead of OkHttp Call
  -h, --help       Show this help message and exit.
  -V, --version    Print version information and exit.