Debug Protocol: Support step-into-specifc

[jacdavis]

In order to use the vscode protocol in vs and xamerin, we'd need to support Step-into-specific. This is a feature that allows the user to select which call a step in should occur on when there are multiple calls on a line. Users primarily use this when there are a lot of calls on a single line (either implicit or otherwise).

This is a trivial example:

The apis around this would require the debug adapter to provide a list of calls at a specific line. The user would then select one. The target call would then be placed as an optional field in the step-in request.

[weinand]

@jacdavis sounds similar to #28, correct?

[gregg-miskelly]

@weinand not really. Set next statement alters the instruction pointer in an artificial way. Step-into-specific is a special kind of step request which does a combination of step-over and step-into to only step-into one specific call site.

[weinand]

@gregg-miskelly @jacdavis @DavidKarlas

I've created a proposal for this protocol addition as PR 44.

Please review and comment.