Pin the default client image with digest (#227)

This changes the default client image from latest to a pinned digest. It
also enforces that the default image version tag matches the version of
the Netbird dependency.

This makes testing a lot easier and also ensures that we wont get
untested behavior introduced if the client makes a breaking change.

Signed-off-by: Philip Laine <philip.laine@gmail.com>

internal/version/version.go

@@ -0,0 +1,5 @@
+package version
+
+func ClientImage() string {
+	return "ghcr.io/netbirdio/netbird:0.70.4@sha256:3a28b9f7f32875c6a35f952ca7e9cb688b1e610365365ff55f6e790da3950f55"
+}