Add datetime.parse and string.format C# samples

[gewarren]

These samples were accidentally deleted in #2064.

[BillWagner]

I agree that we should restore these. But, should they now be in the dotnet-api-docs repo instead?

[gewarren]

I agree that we should restore these. But, should they now be in the dotnet-api-docs repo instead?

I'll do whatever is recommended by you and @Thraka. These snippets already exist in the API docs repo, but in a slightly different format. For example, this snippet in the API docs repo is encompassed in this file that's part of the sample project. So in the API repo, they exist as standalone files without a project file. In this format that I need to resurrect somewhere, the snippet tags aren't included and various snippets are combined into a runnable program with command line UI.

[gewarren]

@Thraka Do you think I should move this into the API repo instead, even though there will be some duplication in the code?

[Thraka]

This looks complete, it's designed as a sample and not as a snippet. I think we should restore it and convert it to a true sample.

[gewarren]

@BillWagner or @Thraka Does this look good? Can you approve if so?

[BillWagner]

This LGTM @gewarren

Let's