refactor: functional options

[pd93]

Description

This PR contains some minor refactoring that I've pulled out of another branch to make reviewing a bit easier.

• Return concrete struct instead of interfaces (as per Go best practice)
• Merge NodeBase and Reader functional options for certificates
  • These are in the same package and can reuse the same functional option. The option struct simply needs to implement both interfaces. This is much easier now that the options return their raw types instead of having to create a composite interface.
• Moves the functional options to their own file
  • This allows the Reader/BaseNode files to focus on actual functionality and not the functional options boilerplate. Since some options can be applied to more than one struct it doesn't make sense to put them in specific files.

Checklist

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