feat: expose fetch option on ManagementClient

[tsushanth]

Why

Closes #1330.

The internal fetcher layer added `BaseClientOptions.fetch?: typeof fetch` in #1238 and routes it through `fetcherImpl(args.fetchFn ?? …)`. The public wrapper, however, both omitted `fetch` from `ManagementClientOptions` and explicitly deleted it from the incoming options bag, so library consumers had no supported way to provide their own HTTP transport — even though the SDK is already capable of using one. Typical use cases the reporter and downstream Auth0 users have called out: proxies / corporate egress, retry middlewares, OpenTelemetry instrumentation, and Cloudflare Workers / Bun targets where the global `fetch` needs swapping.

What

• Drop `"fetch"` from the `Omit<FernClient.Options, …>` list so the field is part of the public `ManagementClientOptions` surface.
• Stop deleting `_options.fetch` in the `ManagementClient` constructor. The internal `fetcher` is still stripped (it's a Fern implementation detail), with a comment explaining why and linking back to this issue.

Tests

`tests/management/ManagementClient.test.ts` — two cases:

1. Constructs a `ManagementClient` with a custom `fetch` implementation, fires `client.rules.list()`, asserts the custom function was invoked at least once and that the URL targets `tenant.auth0.com/api/v2/rules`. Without the wrapper change this fails because `fetch` is silently dropped before reaching `fetcherImpl`.
2. Constructs a `ManagementClient` without `fetch` to confirm the default global-fetch route is unchanged.

`npx jest tests/management/ManagementClient.test.ts` — both pass. `npx tsc --noEmit` clean. `eslint` clean.

Compat

Source-compatible: existing call sites that didn't pass `fetch` see no behavioural difference. Callers can now opt in:

```ts
new ManagementClient({
domain: 'tenant.auth0.com',
token: '...',
fetch: myInstrumentedFetch,
});
```