refactor: update toucan URI to be optional

[ryanchristo]

Description

The toucan URI will most likely be empty when we are ready to launch. In Regen Ledger v5.0, we have implemented the ability to update the metadata for an open credit batch, so we will be able to populate it in the future, but all post requests from the bridge service will fail if we require the URI to be populated. This pull request updates the toucan URI to be optional.

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[clevinson]

utACK on this. We should be treating toucanURI as optional in both project & vintage json-ld's.