Add support for struct tag name overrides in native types

[patrickpichler]

In order to customize how to fields of native types can be accessed via CEL, struct tag support has been added. It is now possible to provide a struct field tag, that will act as the name for the corresponding field.

Here an example

type Person struct {
  Name string
  Age  int `cel:"age"`
}

and here is how to access the Age field from CEL:

person.age

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[TristonianJones]

/gcbrun

[TristonianJones]

Thanks @patrickpichler! This seems like a very nice win.

[quantumsheep]

Wow I just discovered this PR after creating #942! My PR fixes an issue not fixed in yours but completely related to the same use-case

[patrickpichler]

@TristonianJones I've updated the code, would be nice if you could have another look :)

[TristonianJones]

/gcbrun

[TristonianJones]

@patrickpichler thanks for the contribution!