Support NATS as a datastore

[thecodeassassin]

NATS is an excellent datastore for Kamaji now the latest rewrite in kine has been finished.

k3s-io/kine#194 (comment)

We would love to see support for this in Kamaji.

[prometherion]

I'm definitely on this, however, I'm currently understaffed about time availability.

We abstracted the Datastore interaction:

kamaji/internal/datastore/connection.go

Lines 41 to 56 in 1ccc1d1

	type Connection interface {
	CreateUser(ctx context.Context, user, password string) error
	CreateDB(ctx context.Context, dbName string) error
	GrantPrivileges(ctx context.Context, user, dbName string) error
	UserExists(ctx context.Context, user string) (bool, error)
	DBExists(ctx context.Context, dbName string) (bool, error)
	GrantPrivilegesExists(ctx context.Context, user, dbName string) (bool, error)
	DeleteUser(ctx context.Context, user string) error
	DeleteDB(ctx context.Context, dbName string) error
	RevokePrivileges(ctx context.Context, user, dbName string) error
	GetConnectionString() string
	Close() error
	Check(ctx context.Context) error
	Driver() string
	Migrate(ctx context.Context, tcp kamajiv1alpha1.TenantControlPlane, target Connection) error
	}

It would be great if we could have some NATS experts helping in understand the required APIs to call, and I guess the etcd Kamaji's implementation could help here,

[prometherion]

I'm happy to share we will support NATS starting from v0.6.0, even tho with some minor limitations.
Opening an issue to track these.

Thanks a lot @thecodeassassin ❤️