fix: cache definitions in optimizer

[yergom]

Only 1 subtype of an interface types (e.g Node) is optimized. This PR fixes the optimizer's `_interfaces cache, so that all interfaces are considered for optimization. I assume that the cache was introduced to avoid a performance hit (having to iterate over all the schema's types) every time an interface has to be optimized.

Before this PR, the following would happen. Suppose that we have A and B, both implementing the Node interface:

• We resolve node for an instance of A: The _interfaces cache is empty. The optimizer will add to the _interfaces cache only A, which will get all the optimization hints
• We resolve a node for an instance of B: The _interfaces cache contains only A. The optimizer will fail to optimize B.

Types of Changes

• [ ] Core
• [x] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [ ] Documentation

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[codecov-commenter]

Codecov Report

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

Comparison is base (5c999e9) 87.90% compared to head (a4cc6a7) 87.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #474 +/- ## ======================================= Coverage 87.90% 87.90% ======================================= Files 37 37 Lines 3166 3167 +1 ======================================= + Hits 2783 2784 +1 Misses 383 383 ```

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