Add vss_client_v050_compatibility test

[tankyleo]

    Add `vss_client_v050_compatibility` test

    We recently added the requirement that VSS servers return key-version
    lists in creation order. We also added a version header to all VSS
    responses such that clients requiring creation-order lists can ensure
    the VSS server they talk to provides this guarantee.

    These VSS server changes remain fully backwards compatible with
    previously-released VSS clients. Here we add a test that ensures that
    this is true against the last VSS client release made prior to these
    changes.

    This new test also makes sure that any future changes to VSS server
    remain backwards compatible with VSS client v0.5.0.

    AI wrote the test.

[ldk-reviews-bot]

👋 Thanks for assigning @tnull as a reviewer!
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Once they submit their review, I'll check if a second reviewer would be helpful.