Remove Gateway API types from Configuration-related types in dataplane package

[pleshakov]

Proposed changes

Problem:

Configuration-related types in the data plane package include Gateway API types.

As a result:

• We need to make any validation-related assumptions about them (ex. certain fields cannot be not nil) in the config package.
• It makes it difficult maintain/extend and potentially allow for supporting more routing resource types (outside of Gateway API) in the future.
• It makes unit tests more complicated in the config package

Solution:

• Remove Gateway API types from Configuration-related types.
• Move the types to types.go file.
• Move converters to convert.go

Testing: Unit testing

Closes #660

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[kate-osborn]

LGTM! The tests seem much simpler now 🚀

[ciarams87]

🚀