types: Support Set and SetType

[bflad]

Closes #53

[bflad]

I'm going to let things settle with #24, #127, and #128 (all v0.3.0 candidates) before continuing here, personally.

[bflad]

My upfront guess is that this implementation may need to change into allowing duplicates by overwriting existing elements during Set() and SetAttribute() calls instead of returning a duplicate set element validation error. I guess we will want to decide on what the more correct behavior should be -- potentially allowing silently successful duplicates where they may not be intended or potentially causing unnecessary errors with provider implementations that will want the type to just consolidate data automatically.

My first impression is that the current implementation may actually be desirable in this case (and allow the implementation to be much simpler) since practitioners would receive configuration validation errors for duplicate set elements and API fields this is being used against shouldn't have duplicates if this attribute type is being chosen.

I'm not sure if its worth capturing this context anywhere (maybe on the Set/SetType documentation) no matter which way we decide to go.

Edit: As I reread this after submitting it, I'm not exactly sure how we could properly deduplicate only on Set()/SetAttribute() unless we provided some flag to switch the reflection behavior and passed that all the way through. To that end, I'm slightly more convinced this initial implementation may be desirable or we will really need to make some more invasive changes to the framework type handling.

[paddycarver]

Looks good. Some small additions I recommended, and looks like some typos in some tests.

[paddycarver]

Yeah, let's do it. Good work! 👍 🚀

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