Add RegisterIgnoreAttribute to recognize external ignore attributes

[Yinmany]

Summary

Models often already carry framework-level ignore attributes
([JsonIgnore], [BsonIgnore], [NotMapped], ...). Today users must also add
[FastClonerIgnore] to every such member. This adds a global registration API so
FastCloner honors those attributes directly.

API

FastCloner.FastCloner.RegisterIgnoreAttribute<JsonIgnoreAttribute>();

[lofcz]

Hey, thanks for the contribution. I understand the reasoning behing this and consider the need valid. However I don't like the implementation both from performance and ergonomics point of view. Please also disclose the usage of AI - it helps me double check for edge cases.

[lofcz]

Closes #46

[Yinmany]

Thanks for the feedback. Here is the AI usage disclosure:
This PR was implemented end-to-end with Codex assistance (including unit tests), following my instruction to keep the approach as simple as possible. I have manually run the full test suite and verified the code logic.
Your concerns regarding performance and ergonomics are noted — happy to discuss further improvements.

[lofcz]

superseded by #49