fieldpath: add DeleteField function to delete elements from Paved

[muvaf]

Description of your changes

Introduces DeleteField to delete a field from the given object.

Fixes #343

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Unit tests with 100% coverage.

[muvaf]

I've noticed that two cases weren't handled and added them:

• Top-level field could not be removed since iteration assumed there is at least two segments, i.e. accessing i+1. Now we handle this as special case.
• In cases where the given path does not exist even before the last node, i.e. metadata.some.another.other where there is no metadata.some, it returned error. Now it's no-op since the target path is already gone.

Added unit tests for both but couldn't reduce cyclomatic complexity without breaking p.delete into pieces that don't make much sense.

@turkenh could you take another look?

[muvaf]

BTW, I could be convinced to rename this to DeleteKey.

[turkenh]

Key wouldn't be a good fit if array. What about DeleteItem ?

[negz]

Would simply Delete work? As in paved.Delete to delete whatever is at the supplied field path?

[muvaf]

@turkenh I feel like Field is more familiar than Item in this context for the users of fieldpath.

@negz I think it'd read nice if we had Get/Set/Delete but given that there is GetValue/SetValue, paved.Delete feels a bit off without a noun that says what you're deleting.

Thanks for the suggestions folks!