feat: use generics to get messagers

[YoungerLee]

• use generics function to get messagers rather than generate all getter functions
• upgrade go version to 1.18

[wenchy]

Generics may be not friendly to code auto-completion.

[wenchy]

Problem: type parameters in methods

According to the Type parameters proposal, it is not allowed to define type parameters in methods.

Something like this:

type Store struct {}

func (s *Store) Get[model any](key string) (*model, error) {
  ...
}

Currently, having a generic inside a struct function can throw a syntax error like:

syntax error: method must have no type parameters

References

• proposal: spec: allow type parameters in methods golang/go#49085