Reduce hard dependency on curl for sourcemap uploads

## Problem

`faro-cli` (and the bundler plugins that wrap it) shell out to `curl` via `execSync` to POST sourcemaps to the Faro API — see [`packages/faro-cli/src/index.ts:142`](https://github.com/grafana/faro-javascript-bundler-plugins/blob/main/packages/faro-cli/src/index.ts#L142). If `curl` is not on the user's `PATH`, uploads fail with a cryptic shell error.

Environments where this commonly bites:
- Older Windows (curl.exe ships with Windows 10 1803+, but earlier versions and some stripped images don't have it)
- Minimal / Alpine / distroless Docker images used in CI
- Some serverless / build-sandbox environments

## Options to consider

These aren't all mutually exclusive — we could mix and match.

1. **Replace the upload path with Node's built-in `https` / `fetch`.** Removes the dependency entirely. Would need to preserve `--proxy` / `--proxy-user` (e.g., via `https-proxy-agent`) and the `HTTPS_PROXY` / `NO_PROXY` env-var behavior that curl provides for free.

2. **Add an opt-in flag (e.g., `--use-node-http`).** Keep curl as default, let users without curl opt in. Smallest behavior change, but adds a config surface and requires users to know about the flag before they hit the failure.

3. **Auto-detect curl and fall back to Node HTTP.** Best UX (no flags, no surprises), but two upload paths to maintain.

4. **Fail fast with a clear error when curl is missing.** Orthogonal to the above — even if we keep curl, a startup `curl --version` probe could produce an actionable error (\"curl not found on PATH; install curl or use ...\") instead of a generic `execSync` failure.

5. **Postinstall warning.** Emit a warning at `npm install` time if `curl` isn't on `PATH`, so users learn about the requirement before their first build fails.

6. **Improve documentation.** Explicitly call out the curl prerequisite in the README, list known environments where curl isn't shipped, and link to install instructions. Cheapest option; doesn't help users who hit the error before reading the docs.

## Note

The `generateCurlCommand` function (which prints a curl command for users to copy/paste) is a separate use case from the runtime upload — it should remain as-is regardless of which option we choose for the upload path.