feat: generic L2 forwarder base contract

[bmzig]

This PR adds the abstract contract ForwarderBase which is meant to be a base for forwarders. Forwarders should relay messages/tokens from an L2 to an L3. A forwarder which inherits ForwarderBase will have to be constructed for each chain to account for an L2's message validation logic, but since ForwarderBase makes use of L2 adapters, it shouldn't matter what the architecture of the L3 is. To be clear, only _requireAdminSender, an internal function which ensures that the caller is the aliased hub pool, must be implemented for each new forwarder.

[mrice32]

Looks great! A few comments

[nicholaspai]

Some comments on naming. You shouldn't take naming lightly, it's the difference between code that is easy to review and code that's not, and that's a major difference in code quality.

[pxrl]

This PR adds the contract ArbitrumForwarderInterface which is used to send messages/tokens from an arbitrary L2 to an Arbitrum-like L3. The functions which will need to be implemented for the forwarder contracts will be:

• relayTokens: an external function callable by anybody which bridges tokens to the L3 spoke pool.
• _relayMessage: an internal function which sends a message to a specified target on L3.
• _requireAdminSender: an internal function which ensures that the caller is the aliased hub pool.

In general, relayTokens and _relayMessage will be taken from the current Arbitrum_*_Adapter code. Under the assumption that the L2 does aliasing differently than adding 0x11...000...11 to the msg.sender, _requireAdminSender will have to be modified accordingly.

@bmzig This description probably needs an update to reflect the changes that were made during review.

[pxrl]

OK from me!