Obligation extension snapshot behavior should consider the obligation key

[cmoesel]

The obligation extension's name element has a comment saying this:

In a derived profile, if no name is present or the name doesn't correspond to an obligation name found on that element in the base profile, the obligations will be considered additions to rather than replacements of the existing obligations

SUSHI does not currently perform any special behavior for obligation extensions -- but based on this, it should. See also this Zulip thread: #shorthand > Sushi merging profile extension with parent content @ 💬.

Note that some IGs may already depend on or expect the current SUSHI behavior, so we'll want to run regressions to see how making this change affects existing IGs.