Use dedicated type for representing the Group

[jorgebay]

We should use a type for representing the group name that way we make sure that it doesn't get mixed with other string parameters.

For example:

type Group string

and then, use the Group type for method signatures:

type OffsetState interface {
	Initializer
	fmt.Stringer

	// Here: Use Group instead of string
	Get(group Group, ...) offset *Offset

	// Use Group instead of string
	GetAllWithDefaults(group Group, t...) []Offset

	// Use Group instead of string
	Set(group Group, ...) bool
	// ...
}

[jorgebay]

We use mockery for test mocks. Changing the signature of interfaces will require the mocks to be updated, for example:

mockery -r --case=snake \
  --output=./internal/test/interbroker/mocks --name=Gossiper

mockery -r --case=snake \
  --output=./internal/test/types/mocks --name=OffsetState

mockery -r --case=snake \
  --output=./internal/test/data/mocks --name=Datalog