fix(backend): ignore unknown fields for pb json unmarshaling

[HumairAK]

Description of your changes:

When new proto api fields are added to the spec, these break the driver due to unknown field constraints. For example:

Today the driver understands this kubernetes execution config:

platforms:
  kubernetes:
    deploymentSpec:
      executors:
        exec-some-component:
          configMapAsEnv:
          - configMapName: my-configmap    # this exists in the api today

Now suppose we are on kfp 2.4, and in 2.5 we add support for a new field, deprecated the old field configMapName:

platforms:
  kubernetes:
    deploymentSpec:
      executors:
        exec-some-component:
          configMapAsEnv:
          - configMapName: my-configmap           # this is now deprecated and configNameParameter is preferred
            configNameParameter:
              runtimeValue:
                constant: my-configmap

And we add backend support for the new field in 2.5, and then the functional sdk changes. The pipeline compiled using the new sdk will not work for 2.4 because 2.4 drivers will not be able to unmarshal this field configNameParameter because it does not recognize it. This change will allow older versions to just drop this field, making new api additions more friendly to older kfp versions.

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[hbelmiro]

/lgtm

[mprahl]

/lgtm

[HumairAK]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HumairAK, mprahl

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