Add ability to update the curator in a basket

[ryanchristo]

Summary

The creator of a basket is assigned as the curator of the basket upon creation. At the moment, there is no way to update the curator address to another account. A curator should have the ability to reassign the curator role to another account.

Problem Definition

A curator of a basket may want to update to a group account or reassign the curator role to another account.

Proposal

• Add UpdateCurator message to ecocredit.v1.basket message server.
• Add EventUpdateCurator to ecocredit.v1.basket events.
• Add update-basket-curator command to x/ecredit/basket/client.
• Ensure all added functionality has corresponding unit and integration tests.

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