Make async runtimes optional

[nstinus]

Hi,

I successfully integrated lapin with the tokio runtime but I noticed it was still pulling 2 different versions of async-io. Is there a way to make that optional?

$ cargo tree -p lapin -i async-io
error: There are multiple `async-io` packages in your project, and the specification `async-io` is ambiguous.
Please re-run this command with one of the following specifications:
  [email protected]
  [email protected]
$ cargo tree -p lapin -i [email protected]
async-io v1.13.0
└── async-reactor-trait v1.1.0
    └── lapin v2.3.4
        └── my_app v0.1.0 (...)
$ cargo tree -p lapin -i [email protected]
async-io v2.3.2
└── async-global-executor v2.4.1
    └── async-global-executor-trait v2.1.0
        └── lapin v2.3.4
            └── my_app v0.1.0 (...)

Thank you!