allow a data provider to specify a different certificate used in signing a direct result

[stevenwarejones]

A Data Provider may not want to use the same certificate that was noted in the Requisition for various reasons. This update allows a Data Provider to specify a different certificate to verify a Measurement Result.

[wfa-reviewable]

This change is 

[SanjayVas]

Thanks for this! I didn't realize we weren't already taking in the certificate here and were instead using the one associated with the encryption public key.

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src/main/proto/wfa/measurement/api/v2alpha/requisitions_service.proto line 141 at r2 (raw file):

  // Resource name of a `Certificate` belonging to the `DataProvider` that
  // produced the result. If unspecified, the certificate from the
  // data_provider_certificate from the `Requisition` will be used.

nit: I assume you didn't mean to repeat the phrase

Suggestion:

  // produced the result. If unspecified, the `data_provider_certificate`
  // from the `Requisition` will be used.

[SanjayVas]

See #188

[stevenwarejones]

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src/main/proto/wfa/measurement/api/v2alpha/requisitions_service.proto line 143 at r3 (raw file):

  // `data_provider_certificate` from the `Requisition` will be used.
  string certificate = 4
      [(google.api.resource_reference).type = "halo.wfanet.org/Certificate"];

based on PR187, I should change this to required I'm thinking @SanjayVas

[Marco-Premier]

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[SanjayVas]

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src/main/proto/wfa/measurement/api/v2alpha/requisitions_service.proto line 143 at r3 (raw file):

Previously, stevenwarejones (Steven Ware Jones) wrote…

based on PR187, I should change this to required I'm thinking @SanjayVas

The current (informal) API change policy has been to avoid breaking EDP requisition fulfillment in a single release. I don't think we can safely require this until after 0.5.0. Noting that it may be required in the future would be good for now.

[Marco-Premier]

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