Adding composed.To() method

[BigGold1310]

Description of your changes

Adds the composed.To() method to allow an easy conversion from a function-sdk-go Unstructured to a concrete/strongly typed object.

Fixes #93

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

Code is tested by unit-testing.

[negz]

Thanks @BigGold1310!

Could you provide a little more context around how you'd expect to use this function? Is the idea that you'd convert observed composed resources to their real types?

When a function deserializes the Protobuf request the resources are a *structpb.Struct. Behind the scenes we're already using resource.AsObject to convert the Struct to a *composed.Unstructured when you call request.GetObservedComposedResources. Part of me wonders whether we should make it possible to go straight from Struct -> your desired type and skip the Unstructured in the middle, to avoid a layer of serialization. Not a blocker for this PR though given there's already the From function here that has a similar issue.

[BigGold1310]

Thank you @negz for taking a look at this PR.

The idea of this function is to make it easier writing functions with strongly typed objects. So the function should help converting from the composed.Unstructured to a structured resource.

Converting the object directly from the *structpb.Struct sounds reasonable. I would suggest that we look at that implementation in a separate PR as more parts of the code need to be touched.
I'm happy to give it a try if you could provide me some guidance/pointers on where to look at/where to start.