refactor(libstore): minimize NIX_WITH_S3_SUPPORT scope to auth only

Move S3 URL parsing, store configuration, and public bucket support
outside of NIX_WITH_S3_SUPPORT guards. Only AWS credential resolution
remains gated, allowing builds with withAWS = false to:

- Parse s3:// URLs
- Register S3 store types
- Access public S3 buckets (via HTTPS conversion)
- Use S3-compatible services without authentication

The setupForS3() function now always performs URL conversion, with
authentication code conditionally compiled based on NIX_WITH_S3_SUPPORT.
The aws-creds.cc file (only code using AWS CRT SDK) is now conditionally
compiled by meson.

src/libstore-tests/s3-binary-cache-store.cc

@@ -1,12 +1,9 @@
 #include "nix/store/s3-binary-cache-store.hh"
+#include "nix/store/http-binary-cache-store.hh"
+#include "nix/store/filetransfer.hh"
+#include "nix/store/s3-url.hh"
 
-#if NIX_WITH_S3_SUPPORT
-
-#  include "nix/store/http-binary-cache-store.hh"
-#  include "nix/store/filetransfer.hh"
-#  include "nix/store/s3-url.hh"
-
-#  include <gtest/gtest.h>
+#include <gtest/gtest.h>
 
 namespace nix {
 
@@ -126,5 +123,3 @@ TEST(S3BinaryCacheStore, parameterFiltering)
 }
 
 } // namespace nix
-
-#endif