debug: allow debugAdapter=dlv-dap with remote attach mode

[polinasok]

If debug adapter is not specified, still default to legacy for remote attach in case users haven't updated their tools to newer version of dlv that supports this via DAP. We can flip the default in a couple of months. In the meantime, users will need to specify directly if they want to use dlv-dap adapter. There is no good way to detect the version of a running server, so we will always allow them to proceed, while providing a warning about the right version at session start-up and at shutdown via Go Debug output channel if we detect that the connection failed with no responses.

After this change, we will have the following behavior:
attach + remote + debugAdapter= => legacy
attach + remote + debugAdapter=dlv-dap => dlv-dap + version warnings
attach + remote + debugAdapter=legacy => legacy

attach + !remote + debugAdapter=dlv-dap + port => warning to drop port if not using external server
launch + !remote + debugAdapter=dlv-dap + port => warning to drop port if not using external server
launch + remote + debugAdapter=dlv-dap + port => error from dlv server

Updates #1861

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Patch Set 2:

(3 comments)

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Message from Polina Sokolova:

Patch Set 3:

(3 comments)

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