feat(domain): Instance APIs with relational tables (#10953)

# Which Problems Are Solved

This PR implements the endpoints listed in
https://github.com/zitadel/zitadel/issues/10443 .

# How the Problems Are Solved

The implementation follows the same pattern as the Organization one.
There are no peculiarities to this PR. The `Update`, `Delete` are
implementing the `Commander` interface, while `Get` and `List`
instance(s) endpoints are implementing the `Querier` interace.

# Additional Context

- Closes #10443
- Depends on #10445

backend/v3/api/instance/v2/convert/instance.go

@@ -0,0 +1,52 @@
+package convert
+
+import (
+	"google.golang.org/protobuf/types/known/timestamppb"
+
+	"github.com/zitadel/zitadel/backend/v3/domain"
+	"github.com/zitadel/zitadel/cmd/build"
+	instance "github.com/zitadel/zitadel/pkg/grpc/instance/v2beta"
+)
+
+/*
+ * Domain Model to GRPC v2
+ */
+
+func DomainInstanceListModelToGRPCResponse(instances []*domain.Instance) []*instance.Instance {
+	toReturn := make([]*instance.Instance, len(instances))
+
+	for i, inst := range instances {
+		toReturn[i] = DomainInstanceModelToGRPCResponse(inst)
+	}
+
+	return toReturn
+}
+
+func DomainInstanceModelToGRPCResponse(inst *domain.Instance) *instance.Instance {
+	return &instance.Instance{
+		Id:           inst.ID,
+		ChangeDate:   timestamppb.New(inst.UpdatedAt),
+		CreationDate: timestamppb.New(inst.CreatedAt),
+		State:        instance.State_STATE_RUNNING, // TODO(IAM-Marco): Not sure what to put here
+		Name:         inst.Name,
+		Version:      build.Version(),
+		Domains:      domainInstanceDomainListModelToGRPCResponse(inst.Domains),
+	}
+}
+
+func domainInstanceDomainListModelToGRPCResponse(dms []*domain.InstanceDomain) []*instance.Domain {
+	toReturn := make([]*instance.Domain, len(dms))
+	for i, domain := range dms {
+		isGenerated := domain.IsGenerated != nil && *domain.IsGenerated
+		isPrimary := domain.IsPrimary != nil && *domain.IsPrimary
+		toReturn[i] = &instance.Domain{
+			InstanceId:   domain.InstanceID,
+			CreationDate: timestamppb.New(domain.CreatedAt),
+			Domain:       domain.Domain,
+			Primary:      isPrimary,
+			Generated:    isGenerated,
+		}
+	}
+
+	return toReturn
+}