Consider making netty-codec-http[2] dependencies optional

[sdeleuze]

While working with r2dbc-postgresql and WebFlux i a Gradle project, I faced a very annoying issue where the exclusions of netty-codec-http and netty-codec-http2 in r2dbc-postgresql break completely WebFlux since it removes the netty-codec-http dependency from the project. We are going to remove for now these excludes from R2DBC but that just a workaround since it will now pull some dependencies it does not need.

Since reactor-netty usage is wider than just HTTP (R2DBC is a good example), my proposal is to make these 2 dependencies optionals and update consumer projects like WebFlux to add these explicitly.

[bsideup]

We plan to shade them, see #734

[violetagg]

Shading is not an option, most probably we will separate the project to submodules.