Provide a solution for Netty version conflicts - grpc-netty requires specific Netty version

**Is your feature request related to a problem? Please describe.**

grpc-netty is compatible only with specific Netty versions. For example, the officially [supported version is 4.1.100.Final at the moment](https://github.com/grpc/grpc-java/blob/master/SECURITY.md#netty).

grpc provided grpc-netty-shaded package, but that's not compatible with jetcd. There are conflicts with `io.grpc.netty.GrpcSslContexts`/`io.grpc.netty.shaded.io.grpc.netty.GrpcSslContexts` and `io.netty.handler.ssl.SslContext`/`io.grpc.netty.shaded.io.netty.handler.ssl.SslContext` classes.

**Describe the solution you'd like**

I'm not sure what solution would be backwards compatible since GrpcSslContext and SslContext classes are exposed in the jetcd client builders.

**Describe alternatives you've considered**

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**Additional context**

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