Provide interface to get the Fee Manager mode

[akolotov]

Related to Issue 119 Rewards for bridge validators.

As it was identified in omni/bridge-ui#182, there is a need to provide the Fee Manager mode similar we provide the bridge mode.

If the Fee Manager contains functionality to calculate and distribute the reward only for one-side directed transfer, the method should be defined as:

function getFeeManagerMode() public pure returns(bytes4 _data) {
    return bytes4(keccak256(abi.encodePacked("manages-one-direction")));
}

If the Fee Manager contains functionality to calculate and distribute the reward for transfers in both directions, the method should be defined as:

function getFeeManagerMode() public pure returns(bytes4 _data) {
    return bytes4(keccak256(abi.encodePacked("manages-both-directions")));
}

The corresponding wrappers should be provided on the bridge contracts side.

[akolotov]

Available in the release 2.3.0