Refactor e2e mock server

See https://github.com/MetaMask/metamask-extension/pull/17835/files#r1113045235

Refactor the e2e mock server so we can have granular control over mock requests;

AC:
- [ ] add documentation for the e2e mock server
- [ ] mocks can be set before the extension is installed
- [ ] mocks can be set during the test
- [ ] mocks can be specified locally for single tests
- [ ] mocks can be specified globally for all tests
- [ ] you can make assertions on an individual mock

Start with the files and discussion linked from PR #17835 to find the current e2e mock server entry point. Review how mocks are configured now, then use the acceptance criteria to verify setup timing, local and global scope, assertions, and documentation are all covered.

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