chore: fix repo imports and add devcontainer

[mfw78]

Description

Recently some upstream submodules were deleted which resulted in the head branch breaking.

Specifically, these were:

• https://github.com/rndlabs/cowswap-contracts
• https://github.com/rndlabs/balancer
• https://github.com/rndlabs/canonical-weth

These are mostly used in the course of testing. There have also been issues caused in other development associated with ComposableCoW (a.k.a programmatic orders framework) with the large amount of dependencies / git submodule treatment of such.

Changes

• Added a devcontainer for quality of life improvement
• Removed aforementioned submodules
• Added new cowprotocol module targeting main branch of upstream to take advantage of contracts already having had their pragma bumped 🚀
• Refactored all contracts to make all imports explicit - eliminating annoying IERC20 conflicts
• Removed safe module referencing https://github.com/rndlabs and now targets https://github.com/cowdao-grants/extensible-fallback-handler

How to test

1. CI/CD
2. Observe no logic changes in code - only specific import changes

[mfw78]

Merging with 1x approval due no logic changes (imports only)