Add "sendInvalidate" request

[sbobko]

This PR adds support for OpenDebugAD7 to handle sendInvalidate requests which will respond if it successfully fired an InvalidatedEvent to the UI.

Here is the schema:

"SendInvalidateRequest": {
    "allOf": [
        {
            "$ref": "#/definitions/Request"
        },
        {
            "type": "object",
            "properties": {
               "areas": {
                  "type": "array",
                  "description": "Set of logical areas that got invalidated. This property has a hint characteristic: a client can only be expected to make a 'best effort' in honoring the areas but there are no guarantees. If this property is missing, empty, or if values are not understood, the client should assume a single value `all`.",
                  "items": {
                     "$ref": "#/definitions/InvalidatedAreas"
                  }
               },
                "threadId": {
                    "type": "int",
                },
                "stackFrameId": {
                    "type": "int",
                },
            },
            "required": [
                "areas"
            ]
        }
    ]
},

[iAbadia]

I'm interested in having this PR merged

[sbobko]

Hello @iAbadia , Unfortunately, I'm busy right now and I can't spare the time. You can use the code for inspiration and make your own contribution.

[iAbadia]

Thanks @sbobko , I'll do that!

[WardenGnaw]

LGTM Thanks for the contribution. Once the PR checks pass I'll check it in and it should show in the cpptools changelog when it is in.