TargetSource CRD changes

[mcdillson]

this PR introduces a provider key within the TargetSource spec, so that we're able to differentiate which loader is needed based on the configured provider. there can only be one provider, and the profile field is made compulsory as well (not necessarily the final decision to implement it like this)

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[mcdillson]

@karimra I went over the kubebuilder validation fields.

• a combination of MinProperties and MaxProperties didn't work for the provider spec, since this is only for array types. this is now solved with ExactlyOneOf
• if strings need to be specified and cannot be empty, they have to be annotated with MinLength, because otherwise it's only checked for existence, which it does in all cases, regardless if you add omitempty or not

the two provider specs are just placeholders for now, but show how others would need to be added.