Add the ability for ex_data to have a priority

Where an object has multiple ex_data associated with it, then we free that
ex_data in order of priority (high priority first).

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14991)

crypto/encode_decode/encoder_meth.c

@@ -76,6 +76,7 @@ static void *encoder_store_new(OSSL_LIB_CTX *ctx)
 
 
 static const OSSL_LIB_CTX_METHOD encoder_store_method = {
+    OSSL_LIB_CTX_METHOD_DEFAULT_PRIORITY,
     encoder_store_new,
     encoder_store_free,
 };