Hardcoded jsonrpsee version to alpha3

[akru]

Currently, Polkadot crates use jsonrpsee >=0.2.0-alpha5 but subxt is fixed on alpha3. It makes it impossible to use subxt and polkadot in the same project.

It would be very helpful to upgrade the used jsonrpsee version up to the latest.

[niklasad1]

There is a pending PR for that, #266

This awkwardness should go away once we release 0.2 of jsonrpsee