[Composition] Adding string transform ToUpper/ToLower

[mcbenjemaa]

Description of your changes

Adding 2 fields to the StringTransform, in order to make it possible to add extra functionally of the string transformation.
In our case we need to add a convert ToUpper/ToLower.

• string.type field is required and so, every time must be set! (breaking change!!) any thoughts ?
• string.fmt used to format string as usual.
• string.convert accepts 2 values ToUpper, ToLower, will convert the string according to it's value.

type StringTransform struct {
	Type StringTransformType `json:"type"`
	Format *string `json:"fmt,omitempty"`
	Convert *StringConversionType `json:"convert,omitempty"`
}

Fixes #2579

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

cc @negz
/label "backport release-1.5"

How has this code been tested

Tested locally using kind cluster.

[negz]

A few nits, but in general this looks great. Thank you!

[negz]

Two nits and a question that would be nice to fix before we merge, but mostly this looks great. Thanks @mcbenjemaa!

[negz]

Thank you for working on this @mcbenjemaa! One final thing - could you add a section for this new transform to the docs? Apologies that I didn't think to mention this earlier.

Code looks good but I'd like to wait for docs too.