Adds an AddUrlGroup extension method where Uri is provided by IServiceProvider

[rwwilden]

What this PR does / why we need it:
In some cases, the Uri for a UriHealthCheck is available from configuration through an IOptions<...> instance. It should be possible to use this IOptions<...> instance by retrieving it from the IServiceProvider instead of pulling the Uri directly from config yourself.

Which issue(s) this PR fixes:
No issue was reported.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:
No, just an additional UriHealthCheck registration possibility.

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[unaizorrilla]

Hi @rwwilden

Merged, thanks for contribute this, a release build is on appveyor, when finished a new package 3.1.2 will be available on NuGet.