Consider refactoring plugin code in services/chain

[aljo242]

With the addition of a true Plugin system (#2877), we have some a confusing concept in the codebase with the same name.

The idea of this system seems to have been a "plugable" system where different versions of chains could be configured using different logic. With a unified set of logic since Launchpad chains have been deprecated, and the push to remove all associated code from cli in #3033, we should reconsider how this code works - especially because it is much less of a "Plugin" system than what we added in #2877.

Right now, type Plugin is an interface but we only have one implementation for it (stargate plugin). It is very likely that there will only be one implementation from now on, but keeping it as an interface allows us to be flexible in the future.

I suggest that we keep the interface and mainly change the naming:

plugin -> runtimeConfig

From there, some of the logic existing in services/chain/plugin-stargate.go can be removed since it is no longer necessary. Example: func SupportsIBC() is no longer needed because all versions support IBC.

[lumtis]

I'm would be more in favor of removing it personally