Create ExchangeCheckpoint protobuf message for Kingdom-less panel match

[robinsons]

No description provided.

[wfa-reviewable]

This change is 

[stevenwarejones]

in the design, I can't remember if this was put in a signedMessage or not, but I think it should be when its shared

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @robinsons)

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src/main/proto/wfa/panelmatch/client/internal/exchange_checkpoint.proto line 41 at r1 (raw file):

  google.type.Date exchange_date = 3;

  // Fingerprint of the workflow being executed. The fingerprint is of the

nit: cryptographically secure fingerprint

[robinsons]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @robinsons)