`BootstrapBuilder` should be defined as a provider

[Jire]

BootstrapBuilder while possible to configure some params, does not allow the functionality to entirely create the ServerBootstrap yourself by passing in already-existing EventLoopGroups. This means that when trying to utilize multiple RSProt NetworkService in a single application, you cannot re-use your EventLoopGroup, leading to poor performance.

Something like a BootstrapProvider interface should be introduced, allowing the API user to provide their own ServerBootstrap if they choose. BootstrapBuilder would then be renamed BootstrapProviderBuilder and provide from the build result.