debug: debugAdapter2 must accurately communicate dlv-dap capabilities

[polinasok]

This is branching out from #781, which, in addition to support in debugAdapter, requested that 'supportsConditionalBreakpoints' is correctly reflected in debugAdapter2 as well. This is the only capability right now that is not captured, but there will be more going forward. Simply flipping the bit as soon as the related change is made in the delve repo won't work because by default users work with a stable version of delve. To have an accurate reflection of the capabilities, we can:

• wait for a stable delve release to have them and update manually, which might be a long time, on the order of months, depending on the timing of the last release - not a realistic option
• confirm/enforce/download an experimental version of delve (debug: require a dev version of delve when operating in Delve DAP mode #794) and only set the capabilities that this version supports, keeping them in sync manually
• detect the supported capabilities dynamically by forwarding the request to dlv (debug: forward initializeRequest to delve dap to verify capabilities #822)

[gopherbot]

Change https://golang.org/cl/290449 mentions this issue: src/debugAdapter2: remove thin adapter for dlv-dap