Allow merging multi-version CRDs into a single schema

[clux]

Implementation for #569. Follows official docs.

• example showing how upgrades work in practice without conversion
• tests doing a simple yaml schema merge
• docs; how to maintain/upgrade versions with kube-derive

Docs touch on #865 because it's hard to recommend multi-versioning without it, but users can hack around it.
Tests add some dependencies use elsewhere in project (assert_json_diff from examples).

The example is the most interesting, found out this with 2 versions:

• while v1 is active (storage: true); writes to v2 will truncate schema as per v1 defn
• while v2 is active (storage: true); writes to v1 will truncate schema as per v2 defn

this means that you can end up:

• not being able to read from the v1 api if v2 is not backwards compat
• not being able to read from v2 api if v1 is not forwards compat

tried to clarify this as well as possible in the example and docs.

[codecov-commenter]

Codecov Report

Merging #889 (cc3c4ad) into master (98c0ef2) will increase coverage by 0.09%.
The diff coverage is 82.92%.

@@            Coverage Diff             @@
##           master     #889      +/-   ##
==========================================
+ Coverage   70.25%   70.35%   +0.09%     
==========================================
  Files          61       62       +1     
  Lines        4223     4264      +41     
==========================================
+ Hits         2967     3000      +33     
- Misses       1256     1264       +8     

Impacted Files	Coverage Δ
kube-derive/src/lib.rs	0.00% <ø> (ø)
kube-core/src/crd.rs	82.92% <82.92%> (ø)
kube-runtime/src/wait.rs	68.00% <0.00%> (-2.00%)	⬇️

[clux]

This should be pretty ready now. I have made it a separate function rather than implementing it on CustomResourceExt because I'm not doing this for v1beta1, and it's probably not great to add an unimplemented!() on that side.

We could also remove v1beta1 now. Can't find any cloud providers supporting Kubernetes < 1.16 anymore.
EDIT: done in #890

[clux]

@kazk you able to have a look at this?

[clux]

This should be pretty ready now. I have made it a separate function rather than implementing it on CustomResourceExt because I'm not doing this for v1beta1, and it's probably not great to add an unimplemented!() on that side.

Even with v1beta1 removed, I don't think the api is nicer with a static CustomResourceExt::merge_crds so am going to leave this as is as a separate merge_crds fn.