solc-json-interface: do not unconditionally skip serialization of custom keys (#337)

The data structure can be used to build the JSON input for `resolc` too.
In that case serializing of provided custom options should not be
dismissed.

Makes the memory settings struct more modular as a drive-by.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

Cargo.lock

@@ -8705,7 +8705,7 @@ dependencies = [
 
 [[package]]
 name = "revive-solc-json-interface"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "anyhow",
  "rayon",