Support sending messages to multiple bridges.

[tomusdrw]

Follow up of #849

Currently the Call CLI enum, supports BridgeSendMessage variant. However in case there are multiple bridges on one source chain, we need to specify what Target chain we want to send the message to.
The code to generate such message will then need to be target-specific (see #854).

The problem is that target-chain possibilities are specific to the source chain (i.e. not all pairs are valid).
I see two possible solutions for now:

1. Add a generic BridgeInstance parameter, which can either be an instance index of stringified chain name. (both are a bit clunky usage-wise)
2. Change sub-commands that generate calls (send-message, encode-call) to select bridge instead of single network.

I like (2) the most, I think for send-message we already know the target chain, so perhaps just a matter of changing encode-call API.

[tomusdrw]

As Slava noticed the Call enum is also used in the context of MessagePayload, where the it actually signifies a Call-on-the-target chain, not the source chain.
This might not be easy (or possible) to express in the CLI, so perhaps it's a good idea to split the enum into two to prevent a calls, that go like:

Source(MessagePayload) -> Target(Call->BridgeSendMessage) -> Source(Call->Transfer)