Add --proxy and --no-proxy global flags to control HTTP/HTTPS proxy
usage for all network operations (SDK installs, FTC SDK clone/fetch,
Android SDK download).
Proxy resolution priority:
1. --no-proxy → go direct, ignore everything
2. --proxy <url> → use the specified proxy
3. HTTPS_PROXY / HTTP_PROXY env vars (auto-detected)
4. Nothing → go direct
Key implementation details:
- reqwest client is always built through ProxyConfig::client() rather
than Client::new(), so --no-proxy actively suppresses env-var
auto-detection instead of just being a no-op.
- git2/libgit2 has its own HTTP transport that doesn't use reqwest.
GitProxyGuard is an RAII guard that temporarily sets/clears the
HTTPS_PROXY env vars around clone and fetch operations, then restores
the previous state on drop. This avoids mutating ~/.gitconfig.
- Gradle wrapper reads HTTPS_PROXY natively; no programmatic
intervention needed.
- All network failure paths now print offline/air-gapped installation
instructions automatically, covering manual SDK installs and Gradle
distribution download.
Closes: v1.1.0 proxy support milestone
