[confighttp] Make `ToServerOption` into a sealed interface

[mx-psi]

ToServerOption has underlying type func(*privateStruct). I think we should make it into an interface because of:

1. consistency with other modules in this repository
2. allows creating a hierarchy of options (e.g. we could have some Options that work both confighttp and configrpc by creating in the future a shared package for both)

To solve this issue I think we should:

• Add guidance to CONTRIBUTING.md on how to add options
• Make confighttp follow this guidance