* Bump jsonrpsee to v0.15.1

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

* Update cargo.lock

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

* rpc-servers: Adjust RpcMiddleware to WS and HTTP traits

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

* rpc/author: Use `SubscriptionSink`

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

* rpc/chain: Use `SubscriptionSink`

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

* rpc/state:  Use `SubscriptionSink`

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

* rpc/finality-grandpa: Use `SubscriptionSink`

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

* rpc/beefy: Use `SubscriptionSink`

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

* client: Extract RPC string result from queries

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

* Apply rust-fmt

Signed-off-by: Alexandru Vasile <alexandru.vasil...