Add tags / certifications to retirement transactions

[badgley]

Many registries allow individual credits / transactions to carry additional certifications — sometimes called "tags" — which signify that credits meet additional quality standards. For example, credits eligible for use under the international aviation offsetting program, CORSIA, carry a tag denoting that the credits meet that program's standards. Other tags include "CCP" (meaning the credits satisfy the core carbon principles of the ICVCM).

All five registries currently included in OffsetsDB provide per-transaction tags. As expected, however, each registry represents those tags in slightly different ways.

As a starting point, we should consider adding per-transaction tags to the downloadable version of OffsetsDB. There are two possible approaches: normalized and denormalized. For the normalized option, we would add a boolean column for each possible tag. So a transaction that is CORSIA eligible would have a new column like is_corsia_eligible. Under the denormalized approach, we could create a single column called tags or certifications that contains a list of standardized tags. For example, a transaction of credits that are both CORSIA and CCP eligible would have a list of tags like ["CCP", "CORSIA"].

I'm somewhat partial to the normalized approach but that's really just a weak preference.

[housiicvcm]

Note that these certifications are also applied to issuance transactions.