feat: add C03 credit class to standards

[wgwz]

Description

Closes: https://github.com/regen-network/regen-registry/issues/1509

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[wgwz]

note-to-self: i think it makes sense to merge this PR after these two have been merged first:

1. https://github.com/regen-network/regen-registry-standards/pull/37
2. https://github.com/regen-network/regen-registry-standards/pull/40

because of: https://github.com/regen-network/regen-registry-standards/pull/38#discussion_r1099384777