exchange: introduce a '_canusebundle2' function This function refactors the logic that decides to use 'bundle2' during an exchange (pull/push). This will help being consistent while transitioning from the experimental protocol to the final frozen version. I do not expect this function to survive on the long run when using 'bundle2' will become a simple capability check. This is also necessary to allow HG2Y support in an extension to ease transition of companies using the experimental protocol in production (yeah...). Such extension will be able to wrap this function to use the experimental protocol in some case.

bundle2: detect bundle2 stream/request on /HG2./ instead of /HG2Y/ To support more bundle2 formats, we need a wider detection of bundle2-family streams. The various places what were explicitly detecting the full magic string are now matching on the first three characters of it.