Upgrade darling and syn and rename #[kube(struct)]

[clux]

• syn 2
• darling 0.20 (they bumped minors to nearest 10)

Somewhere from this upgrade is a breaking change that makes it illegal to use struct as an attribute name.

Users will see this error if using #[kube(struct)]:

  --> examples/crd_derive.rs:18:5
   |
18 |     struct = "FooCrd",
   |     ^^^^^^

Have worked around this by renaming this attribute name to something less keywordy:

-#[kube(struct = FooCrd)]
+#[kube(root = FooCrd)]

Will point this out in the release notes if it is accepted.

[codecov]

Codecov Report

Merging #1307 (7f1a577) into main (fba2b9d) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1307   +/-   ##
=====================================
  Coverage   72.4%   72.4%           
=====================================
  Files         75      75           
  Lines       6343    6343           
=====================================
  Hits        4586    4586           
  Misses      1757    1757           

Files	Coverage Δ
kube-derive/src/custom_resource.rs	13.8% <ø> (ø)
kube-derive/src/lib.rs	0.0% <ø> (ø)

[nightkr]

How about crd or object? resource_struct maybe...

[clux]

How about crd or object? resource_struct maybe...

All of these feel too overloaded to me. I kind of like something with generate or root (root_struct?) because we refer to the generated struct, generated root type a lot in https://docs.rs/kube/latest/kube/derive.CustomResource.html

[nightkr]

I like root.