perf: cache DID documents (optional)

[daniel-mader]

Description of change

Enables an optional in-memory cache for DID documents (implemented in did-manager). It enhances the speed of interactions for DIDs that need to be resolved from some trusted network.

Enable this feature with caution! Caching DID documents can cause unexpected behavior.

If you decide to enable the cache, you should be aware of the implications and keep the TTL to a minimum.

Links to any relevant issues

n/a

How the change has been tested

Enable the cache with different parameters and exploratively test its behavior.

Definition of Done checklist

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• I have commented my code, particularly in hard-to-understand areas
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• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
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