make fetch implementation pluggable

1. Provide default fetch impl by default
- to make simple case easy, just depend on node-fetch-retry and done
- whatever seems most common, right now still node-fetch v2, soon v3, and then the nodejs native solution
2. Allow to override `fetch()` impl used

See related #75 and #84

Start by reviewing the repository's current fetch implementation and the discussion in related issues #75 and #84. Determine how the default implementation and an override should be exposed; the work is done when both the simple default case and custom fetch implementation are supported.

Written by the indexing model from the issue text.