Add padding to compressed+encrypted files (#15282)

Add up to 256 bytes of padding for compressed+encrypted files.

This will obscure the obvious cases of extremely compressible content 
and leave a similar output size for a very wide variety of inputs.

This does *not* mean the compression ratio doesn't leak information 
about the content, but the outcome space is much smaller, 
so often *less* information is leaked.

internal/bucket/encryption/bucket-sse-config.go

@@ -128,7 +128,7 @@ type ApplyOptions struct {
 // set minimal SSE-KMS headers if autoEncrypt is true and the BucketSSEConfig
 // is nil.
 func (b *BucketSSEConfig) Apply(headers http.Header, opts ApplyOptions) {
-	if _, ok := crypto.IsRequested(headers); ok {
+	if crypto.Requested(headers) {
 		return
 	}
 	if b == nil {