[TLS] Add support for platform-specific CA bundles.

Adds a new OS::get_system_ca_certs method which can be implemented by
platforms to retrieve the list of trusted CA certificates using OS
specific APIs.

The function should return the certificates in PEM format, and is
currently implemented for Windows/macOS/LinuxBSD(*)/Android.

mbedTLS will fall back to bundled certificates when the OS returns no
certificates.

(*) LinuxBSD does not have a standardized certificates store location.
    The current implementation will test for common locations and may
    return an empty string on some distributions (falling back to the
    bundled certificates).

core/os/os.h

@@ -137,6 +137,7 @@ public:
 	virtual String get_stdin_string() = 0;
 
 	virtual Error get_entropy(uint8_t *r_buffer, int p_bytes) = 0; // Should return cryptographically-safe random bytes.
+	virtual String get_system_ca_certificates() { return ""; } // Concatenated certificates in PEM format.
 
 	virtual PackedStringArray get_connected_midi_inputs();
 	virtual void open_midi_inputs();