Engine: Set the `to_aiida_type` as default inport port serializer

[sphuber]

Fixes #5890

The to_aiida_type serializer automatically converts a number of Python base types to the corresponding Data node when passed as an input to a process. Process functions already automatically set this serializer as a default, which simplifies the life of users and developers.

Here, the to_aiida_type is now set as the default serializer for all input ports. The only exception is if the port is a metadata input port, in which case the data is stored directly in the attributes of the process node and so should not be converted to a node. It is also skipped if the port itself already declares a serializer.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.70%. Comparing base (ef60b66) to head (255ee56). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6439 +/- ## ========================================== + Coverage 77.51% 77.70% +0.20% ========================================== Files 560 562 +2 Lines 41444 41726 +282 ========================================== + Hits 32120 32420 +300 + Misses 9324 9306 -18 ```

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