`spin add`: remove trigger compatibility check

[itowlson]

#2027 reinstated a check in spin add that the component being added had the same trigger type as the current ones. This was correct at the time because we then supported only a single trigger type in an app. However, time moves on (spoilers), and an app can now cheerfully (well, moderately cheerfully) have more than one trigger type. So we should remove the check.

Thanks @michelleN for spotting that the check was still there! (I thought I had removed it when we did multi-trigger-type, but I guess that was just a beautiful dream.)

[michelleN]

@itowlson The latest commit in PR #3617 tackles removal of the check for v2 manifests. Does that satisfy this issue in its entirety?

[itowlson]

@michelleN It does - thanks! How would you feel about pulling that out as a separate fix, so it's not coupled to the middleware template discussion? (since this would currently, as far as I can tell, affect someone adding e.g. a cron component to a HTTP app)

[michelleN]

awesome. will do!