This PR aims to add support for rr replay and core actions from the DAP layer. This basically encloses the following:
New launch modes: replay and core

The following modes are added:

    replay: Replays an rr trace, allowing backwards flows (reverse continue and stepback). Requires a traceDirPath property on launch.json pointing to a valid rr trace directory.
    Equivalent to dlv replay <tracedir> command.
    core: Replays a core dump file, showing its callstack and the file matching the callsite. Requires a coreFilePath property on launch.json pointing to a valid coredump file.
    Equivalent to dlv core <exe> <corefile> command.

Dependencies

To achieve this the following additional changes were made:

    Implement the onStepBackRequest and onReverseContinueRequest methods on service/dap
    Adapt onLaunchRequest with the requried validations and logic for these new modes
    Use CapabilitiesEvent responses to enable the StepBack controls on the supported scenarios (see dicussion here)
    Add the corresponding launch.json support on vs code: 

Support for replay and core modes golang/vscode-go#1268