Support 'EvaluateRequest' based data hovers

[weinand]

Currently the VSCode debug UI does not use evaluate requests for feeding data hovers and we do not want to change this implementation in general because it will break existing debug adapters. What we are planning is the following: based on "feature negotiation" (see microsoft/vscode-debugadapter-node#9) a debug adapter can announce that the VSCode debug UI should use the EvaluateRequest for data hovers. In this request VSCode will pass a new context string 'hover' to the debug adapter which implies that the evaluate should not allow side effects.

CC @gregg-miskelly @isidorn

[weinand]

@isidorn the InitializeRequest now returns an attribute supportEvaluateForHovers and the EvaluateRequest accepts a category hover.