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/meson.build

@@ -265,7 +265,6 @@ config_priv_h = configure_file(
 subdir('nix-meson-build-support/common')
 
 sources = files(
-  'aws-creds.cc',
   'binary-cache-store.cc',
   'build-result.cc',
   'build/derivation-builder.cc',
@@ -344,6 +343,11 @@ sources = files(
   'worker-protocol.cc',
 )
 
+# AWS credentials code requires AWS CRT, so only compile when enabled
+if curl_s3_store_opt.enabled()
+  sources += files('aws-creds.cc')
+endif
+
 subdir('include/nix/store')
 
 if host_machine.system() == 'linux'