[WIP] ⚠️ Allow specifying a different credentials per VSphereMachine

[GabQ-Bib]

… and defaulting to this identity if it exists.

What this PR does / why we need it:

This PR adds support for configuring a VsphereIdentity in the vspheremachine / vspherevm objects, which the controller will use when looking for credentials to contact the vcenter with. This appears to be a good solution to the issue mentioned below, because these objects already carry the server name & thumbprint so it makes sense they also carry the credentials.
This PR is a rewrite of the PR originally proposed by farodin91.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1720

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[GabQ-Bib]

I can see several way of implementing the feature:

• Add a second "getCredential" method that is specifically called when trying to extract credentials from the VspereVM (instead of the cluster). This is the implementation you will find in the attached commit. It has the less impact on the codebase because the original method signature is kept throughout the rest of the controllers. As of right now, both method share a lot of code which is not ideal. Some of the code of the two methods could be factored in a 3rd method to help with code duplication.
• A second implementation would be to change the getCredential method signature to directly use a "VsphereIdentityRef" along with a "namespace" string to cover all use cases. It requires a modest refactoring of the controllers that already use this method but it feels ultimately a better approach. I have a commit ready with this implementation.
• I'm wondering if we could just change the method signature to expect an "object that has a property VsphereIdentityRef" which both cluster and vspherevm would comply to, removing the need to change the way the method is called in the rest of the code. My understanding of Go makes me think this would require creating such a type and having both cluster and vspherevm embed it, but It feels like a much more profound rework than the other two implementation proposed above.

I'm still in the process of testing whether this implementation fixes the issue in a satisfactory manner for my use case. In the meantime I would love if you could find the time to do a quick overview of my proposal and provide insights.

[chrischdi]

/retitle [WIP] ⚠️ Allow specifying a different credentials per VSphereMachine