Added Search Client Commands

Added a few commands for viewing the data in the search service:
* `nucmd search query` - Password **not required** - Searches the index
* `nucmd search checksums` - Password **required** - Fetches Package Checksum info from the index
* `nucmd search diag` - Password **required** - Fetches diagnostic info
* `nucmd search fields` - Password **required** - Fetches list of stored fields in index

Fixes NuGet/NuGetGallery#1955