Added Native Token Streaming Enforcer

[hanzel98]

What?

• Added NativeTokenStreamingEnforcer, this enforcer allows to transfer native tokens that accrue with time, and an initial amount.

Why?

• This enforcer allows delegations to be used for streaming services, especially created for native tokens.

How?

• Allows to accrue tokens based on an amountPerSecond, initialAmount and maximumAmount values set by the delegator when creating the terms.

[jeffsmale90]

LGTM

Would we consider sharing some of the implementation between the native and erc20 implementations?

A lot of the logic is identical, until the actual validation against the execution data.