feat: introduce a model manager domain service

The model manager package in internal/jimm is the first for a Juju related domain that moves logic away from the large jimm struct into a dedicated package. This PR shows that the model manager relies on bits from what will eventually be the cloud and controller managers and also requires a way to dial Juju controllers.

This change is incomplete because I'd like the team to get a feel for whether they enjoy this approach of a smaller package with more dependencies OR whether they'd prefer a larger (i.e. single) Juju package that has less dependencies.

internal/jimm/controller_test.go

@@ -21,6 +21,7 @@ import (
 	"github.com/juju/juju/state"
 	"github.com/juju/names/v5"
 	semversion "github.com/juju/version"
+	"github.com/juju/version/v2"
 	"gopkg.in/macaroon.v2"
 
 	"github.com/canonical/jimm/v3/internal/dbmodel"
@@ -1471,6 +1472,12 @@ func TestInitiateMigration(t *testing.T) {
 	}
 }
 
+// newVersion wraps version.MustParse to return a *version.Number
+func newVersion(s string) *version.Number {
+	n := version.MustParse(s)
+	return &n
+}
+
 type result struct {
 	err    error
 	result any

internal/jimm/juju/model/export_test.go

@@ -0,0 +1,4 @@
+package model
+
+// ModelManager is a type alias to export modelManager for use in tests.
+type ModelManager = modelManager