consolidate and simplify passthroughs

[davidangb]

for discussion.

• Consolidate multiple passthrough routes into streamingPassthrough
  • I'd like to mark passthrough as deprecated; perhaps wait until we remove some more usages of it
• Consolidate multiple *ApiService route classes into a single passthrough-only class
  • Makes it easier to see routes at a glance.
  • Eventually, I suggest that all passthroughs exist in this single class, and the only routes left in other classes are non-passthroughs
• Remove unit tests for passthrough-only routes. The streamingPassthrough directive has its own unit tests, so there's not much to test here.
  • this change feels a little squirrely. Should we add back in a unit test to confirm we are routing passthroughs to the correct underlying service?

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