Improve how chainId is handled

[anxolin]

Description

Currently the chainId is passed in the SDK constructor and there's no way to change it either manually or automatically (detect provider's network change). This is cumbersome as the client needs to instantiate one SDK per network, and also it can cause issues when the provider/signer changes the active network.

Solution

Provide a simpler way to create one SDK instance and add a setter to update the chainId and also handlers to react to a provider's change.

Original issue gnosis/cow-sdk#12 by @ramirotw