XXX/lightclient: Use unstable backend

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

subxt/Cargo.toml

@@ -61,7 +61,7 @@ unstable-metadata = []
 
 # Activate this to expose the Light Client functionality.
 # Note that this feature is experimental and things may break or not work as expected.
-unstable-light-client = ["subxt-lightclient", "tokio-stream"]
+unstable-light-client = ["subxt-lightclient", "tokio-stream", "tokio"]
 
 [dependencies]
 async-trait = { workspace = true }
@@ -118,6 +118,8 @@ getrandom = { workspace = true, optional = true }
 # Included if "native" feature is enabled
 tokio-util = { workspace = true, features = ["compat"], optional = true }
 
+tokio = { workspace = true, features = ["macros", "time", "rt-multi-thread"], optional = true }
+
 [dev-dependencies]
 bitvec = { workspace = true }
 codec = { workspace = true, features = ["derive", "bit-vec"] }