Add extensibility point to resolve descriptors in assembly.

- Modified our default TagHelperDescriptorResolver to provide a protected virtual method that it uses under the covers to resolve descriptors for an entire assembly. This will enable tooling to cache descriptors on a per-assembly basis.

#214
Add ability to resolve all TagHelperDescriptors with one method call.

- Modified the AddOrRemoveTagHelperSpanVisitor to no longer manage TagHelperDescriptors found in the system. Instead it now manages TagHelperDescriptorRegistrations which are then used to resolve TagHelperDescriptors.
- 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 TagHelperDescriptors at once and manages descriptors found in the system based on values on the provided TagHelperDescriptorRegistrations.

#214
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 TagHelperDescriptorResolutionContexts.
- Removed several tests from the AddOrRemoveTagHelperSpanVisitorTests. This was due to the change in responsibility of managing the found TagHelperDescriptors; the TagHelperDescriptorResolver now does this.
- Added several new tests to verify the TagHelperDescriptorResolver manages resolved TagHelperDescriptors based on the given TagHelperDescriptorRegistrations.

#214