Enable users to download alternative copies of omnisharp

[rchande]

We should provide capabilities like the CLI does, where we enable downloading of different feeds of omnisharp.

[rchande]

Our new downloading code should also address #1896

[DustinCampbell]

What is the end user scenario?

[rchande]

@DustinCampbell This primarily enhances dev scenarios, but I could see this benefiting users, too:

• As a dev (or user), I can start C# extension for VSCode and have it download a version of OmniSharp other than the version it has hard coded.

This will let us easily set up CI builds for testing the extension with newer/older/etc bits of OmniSharp and also improve the local dev scenario for consuming new OmniSharp in the extension. @jonsequitur also thinks we can leverage this in try.net.

[DustinCampbell]

It might be worth re-titling this issue to reflect that. 😄

[akshita31]

This has been resolved in #2065