Add support for cosmwasm backend

#### Motivation / Context

Cosmwasm [backend](https://github.com/hyperledger-labs/perun-cosmwasm-backend) and [contracts](https://github.com/hyperledger-labs/perun-cosmwasm-contract) have been released for go-perun. Add support for configuring the perun-node to use the cosmwasm backend.

#### Description

Core package of the perun-node (session) is blockchain agnostic. However, components like key management, identity management are specific to each blockchain. So, add implementations for managing keys for the cosmos backend.

Also, update the configuration parameters of the node, so that users can easily choose between ethereum or cosmos backend with simple configuration changes.

##### Relates to

#### Testing

The end-to-end tests for payment API should work for cosmos backend as well.

#### Thread Safety