feat: implement Simple List API for S3 objects

- Add List method with ListInput and ListResponse structs following simples3 patterns
- Implement pagination, prefix filtering, and delimiter grouping
- Add comprehensive test suite for List functionality
- Update tests to use environment variables instead of hardcoded values
- Fix deprecated io/ioutil usage throughout codebase
- Add comprehensive README with API documentation and examples
- Add Nix development environment with flake.nix
- Add Justfile for modern command runner
- Add Docker Compose setup for local MinIO testing
- Update GitHub Actions to use Nix and Justfile for CI consistency
- Fix test naming and setup flow issues

New API:
s3.List(ListInput{
    Bucket: "my-bucket",
    Prefix: "documents/",
    MaxKeys: 100,
})

All tests pass with new simplified API design.

policy.go

@@ -152,7 +152,7 @@ func buildUploadSign(nowTime time.Time, credential string, uploadConfig UploadCo
 	})
 }
 
-func (s3 S3) buildCredential(nowTime time.Time) []byte {
+func (s3 *S3) buildCredential(nowTime time.Time) []byte {
 	var b bytes.Buffer
 	b.WriteString(s3.AccessKey)
 	b.WriteRune('/')
@@ -166,7 +166,7 @@ func (s3 S3) buildCredential(nowTime time.Time) []byte {
 	return b.Bytes()
 }
 
-func (s3 S3) buildCredentialWithoutKey(nowTime time.Time) []byte {
+func (s3 *S3) buildCredentialWithoutKey(nowTime time.Time) []byte {
 	var b bytes.Buffer
 	b.WriteString(nowTime.Format(shortTimeFormat))
 	b.WriteRune('/')