feat(server/v2): add initial implementations of v2 env endpoints

[GeorgeMac]

Supports #3832

Primarily, this brings in the new Environment storage abstraction and a server implementation which interfaces with it:

flipt/internal/server/v2/environments/storage.go

Lines 37 to 52 in 2c989cd

	type Environment interface {
	Name() string
	// Namespaces
	GetNamespace(_ context.Context, key string) (*environments.NamespaceResponse, error)
	ListNamespaces(context.Context) (*environments.ListNamespacesResponse, error)
	CreateNamespace(_ context.Context, rev string, _ *environments.Namespace) (string, error)
	UpdateNamespace(_ context.Context, rev string, _ *environments.Namespace) (string, error)
	DeleteNamespace(_ context.Context, rev, key string) (string, error)
	// Resources
	View(_ context.Context, typ ResourceType, fn ViewFunc) error
	Update(_ context.Context, rev string, typ ResourceType, fn UpdateFunc) (string, error)
	}

The upcoming environment storage implementation will be driven by configuration and the updated git storage backend, which will also come in a subsequent PR.

Additionally, this PR updates the declarative FS snapshot layer to drop the now defunkt storage API implementations.
These were all dependent on the now deleted protobuf definitions.

[markphelps]

looks great!