Add ability to resolve all TagHelperDescriptors with one method call - DTH Communication Part 2

*This PR consists of several different aspects entailed by #214. It essentially covers bullet points 4-5.*

Add ability to resolve all `TagHelperDescriptor`s with one method call.
- Modified the `AddOrRemoveTagHelperSpanVisitor` to no longer manage `TagHelperDescriptor`s found in the system. Instead it now manages `TagHelperDirectiveDescriptor`s which are then used to resolve `TagHelperDescriptor`s.
- Changed the signature of `ITagHelperDescriptorResolver` to take in a `TagHelperResolutionContext` which will allow us to pass more information without breaking tooling.
- `TagHelperDescriptorResolver` now resolves all `TagHelperDescriptor`s at once and manages descriptors found in the system based on values on the provided `TagHelperDirectiveDescriptor`s.

Modify tests to use new `ITagHelperDescriptorResolver` signature.
- Updated all of the tests to use the new `ITagHelperDescriptorResolver` signature so instead of passing strings they now construct `TagHelperDescriptorResolutionContext`s.
- Removed several tests from the `AddOrRemoveTagHelperSpanVisitorTest`s. This was due to the change in responsibility of managing the found `TagHelperDescriptor`s; the `TagHelperDescriptorResolver` now does this.
- Added several new tests to verify the `TagHelperDescriptorResolver` manages resolved `TagHelperDescriptor`s based on the given `TagHelperDirectiveDescriptor`s.

#214

Part 1 PR: #218

/cc @yishaigalatzer @dougbu @pranavkm 
reopening a closed comment: `TagHelperDescriptorResolutionContext`, `TagHelperDirectiveDescriptor`, `TagHelperAttributeDescriptor`, and `TagHelperDescriptor` all go over the same wire and should be consistent.  the two you're adding here (the first two in my list) are not consistent with the existing classes.  given the serialization requirement you mentioned earlier, use `private set` everywhere.

note in particular that `TagHelperDescriptorResolutionContext` is the odd one out.  unlike `TagHelperDescriptor` (and most classes containing collection properties), it does not initialize `DirectiveDescriptors` in its constructor.