feat: enable base fetch override

[penovicp]

Motivation and Resolution

Enables a base fetch override for the channel classes and ensures the same fetch is used within the underlying BatchClient. This is the first step of the isomorphic-fetch and fetch-cookie dependency removal.

Checklist:

• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Linked the issues which this PR resolves
• Documented the changes in code (API docs will be generated automatically)
• Updated the tests
• All tests are passing