Validate OmniSharp payload

[rchande]

#2690 is an example of an issue where the OmniSharp download was either incomplete or did not extract properly. Let's consider adding a mechanism to validate that the download resulted in the correct directory structure.

[rchande]

@akshita31 Any big brainthoughts here?

[gregg-miskelly]

One improvement that I have thought about, but never found the time to implement is to have a SHA 256 (or some other algorithm if you prefer) hash of the dependency in package.json. That way we could easily detect that the downloaded zip is wrong.

[akshita31]

Steps:

• Add the check in the extension for the integrity
• Update omnisharp to produce the integrity check sum