Create lib to handle Weth in routers

[joaobrunoah]

Description

Some routers (like CowRouter) will need to handle Native ETH, but don't need extra features from RouterCommon (like Permit2 or some transient states).

This PR decouples the logic to handle Weth from RouterCommon, so it can be reused by other routers.

Type of change

• Bug fix
• New feature
• Breaking change
• Dependency changes
• Code refactor / cleanup
• Optimization: [ ] gas / [ ] bytecode
• Documentation or wording changes
• Other

Checklist:

• The diff is legible and has no extraneous changes
• Complex code has been commented, including external interfaces
• Tests have 100% code coverage
• The base branch is either main, or there's a description of how to merge

[EndymionJkb]

Great simplification!

[jubeira]

Thanks @joaobrunoah, looks good for the most part! I have a few observations; other than that this makes sense.

[elshan-eth]

Maybe small unit test would be good for this lib but in general LGTM!

[jubeira]

LGTM!