Permit literal string validation for CEL expressions

[Danil-Grigorev]

Motivation

In order to simplify usage of CEL in schema generation with x_kube macro, users interested in simple validation should be able to specify simple &str data without additional syntax, like mandatory .into() call for conversion into Rule type required now.

Solution

This change allows to write simple CEL expressions like so:

#[x_kube(validation = "self.nonNullable == oldSelf.nonNullable")]

as opposed to previous:

#[x_kube(validation = "self.nonNullable == oldSelf.nonNullable".into())]

or

#[x_kube(validation = Rule::new("self.metadata.name != 'forbidden'"))]

and combine multiple more complex expressions with different input types, in case a message or a reason context needs to be specified:

#[x_kube(validation = "self != 'not legal'", validation = Rule::new("self != 'not that'"))]
#[x_kube(validation = ("self != 'also not that'", "some pretty good reason"))]

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.2%. Comparing base (0d79006) to head (6c7fcb6).

Files with missing lines	Patch %	Lines
kube-core/src/cel.rs	66.7%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1754     +/-   ##
=======================================
- Coverage   76.2%   76.2%   -0.0%     
=======================================
  Files         84      84             
  Lines       7883    7889      +6     
=======================================
+ Hits        6003    6006      +3     
- Misses      1880    1883      +3     

Files with missing lines	Coverage Δ
kube-derive/src/cel_schema.rs	93.7% <100.0%> (ø)
kube-derive/src/custom_resource.rs	84.1% <ø> (ø)
kube-derive/src/lib.rs	0.0% <ø> (ø)
kube-derive/tests/crd_schema_test.rs	97.1% <ø> (ø)
kube-core/src/cel.rs	62.5% <66.7%> (-1.2%)	⬇️

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