static void copy(InputStream is, OutputStream os) throws IOException {
        byte[] buf = new byte[4096];
        int b;

        while ((b = is.read(buf)) != -1)
            os.write(buf, 0, b);
    }
Retrieves a JWT access token in its serialized three-part form. The implementation
is free to determine how it should be retrieved but should not perform validation
on the result.

<b>Note</b>: This is a blocking function and callers should be aware that the
implementation communicates over a network. The facility in the
{@link javax.security.auth.spi.LoginModule} from which this is ultimately called
does not provide an asynchronous approach.

@return Non-<code>null</code> JWT access token string

@throws IOException Thrown on errors related to IO during retrieval
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/secured/HttpAccessTokenRetriever.java