Motivation:

Users need a way to change low-level Netty settings, like
`AdaptiveRecvByteBufAllocator` parameters. In the future, we can use
this interface for more similar settings without the need to change
client/server builder.

Modifications:

- Add `TransportConfig` and `TransportConfigBuilder` in `transport-api`
module;
- Add methods on `SingleAddressHttpClientBuilder` and
`HttpServerBuilder` to let users pass `TransportConfig`;
- Wire it up to propagate to `Tcp[Client|Server]ChannelInitializer` as
`TransportConfigInitializer`;

Result:

Users can control `AdaptiveRecvByteBufAllocator` settings.

…/DefaultHttpServerBuilder.java