Refactor edge-config to prepare for Next.js support

This change refactors the edge-config implementation in preparation for supporting Next.js specific builds. The key goal is to prepare the function interfaces where we intend to push the "use cache" boundaries for Next.js support so they are stringly typed. This is to help with optimizing a fast path for "use cache" that can avoid some serialization.

One concession in this refactor is we need to move from a closure for the dev-only SWR edge config fetcher. The problem with this architecture is the function created in the closure won't be accessible to the the individual methods (like get) when they are behind a "use cache" because functions are not serializable. The technique now employed is a lazy created SWR deduper that is keyed off the connection string. This implementation does not consider garbage collection when clients are collected because 1) this is dev only and dev servers don't live very long and 2) this library does not typically require creating arbitrarily many clients since connections are typically stable for the lifetime of a deployment. We could use a FinalizationRegistry but that would limit support to runtimes that provide this and require some careful conditional requiring which just adds bundling complexity.

Author: gnoff