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/s3-url.cc

@@ -1,13 +1,10 @@
 #include "nix/store/s3-url.hh"
+#include "nix/util/error.hh"
+#include "nix/util/split.hh"
+#include "nix/util/strings-inline.hh"
 
-#if NIX_WITH_S3_SUPPORT
-
-#  include "nix/util/error.hh"
-#  include "nix/util/split.hh"
-#  include "nix/util/strings-inline.hh"
-
-#  include <ranges>
-#  include <string_view>
+#include <ranges>
+#include <string_view>
 
 using namespace std::string_view_literals;
 
@@ -117,5 +114,3 @@ ParsedURL ParsedS3URL::toHttpsUrl() const
 }
 
 } // namespace nix
-
-#endif