Extend the extension contracts to support meta transactions

[akolotov]

The idea of this issue is to implement a support of meta transactions on the mediators contract as so bridged tokens (or native tokens permit()) could be triggered for the transfer without the necessity in the direct interaction with the contracts from the user side.

For example, the interaction flow could be like the following:

• the user is requested to sign a message which could be used to approve a particular account to use transferFrom functionality of the token contract
• the message and the signature are incorporated into a call of an extended version of relayTokens/relayTokensAndCall
• some agent sends a transaction with the extended version of relayTokens/relayTokensAndCall to the mediator contract
• the mediator contract uses the message to get the approval for itself to use transferFrom
• the mediator contract picks the token and proceeds with the action to complete the token transfer

[akolotov]

Perhaps it makes sense to consider the issue after the implementation of #21.