Create endpoint `/runoverridetarget`

[david-driscoll]

See #29

We need a way to produce the override snippet in some way.

[david-driscoll]

see #20 for pointers

[DustinCampbell]

@mattwar is ensuring that override completion is going to be exposed by the completion list API in Roslyn. So, we could either augment the existing /autocomplete target, or have something new.

[david-driscoll]

I'm willing to hold out until we see what the completions API gives up, we might have to do some trickery to allow something like...

override $

to transition to

pubic override ToString() {...}