fix(x/ecocredit): impl AminoMarshaler for DateCriteria

[technicallyty]

Description

implements the AminoMarshaler for DateCriteria.

ref: #732

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[aaronc]

We need to actually test the amino JSON it outputs. This is a use case for golden or example tests

[aaronc]

The JSON needs to be human readable. I imagine this will be base64

[clevinson]

Closing this as we're just going to remove the oneof for date criteria.