ref(bundle/*): replace mapstructure tags with yaml tags

[vdice]

Represents a possible dual closing of #91 and adding yaml tags to all of the structs (per recent history of third-party use in #90 and #92; 92 could/would be superseded by this PR.)

[jeremyrickard]

Do we need the YAML? The spec only really supports JSON for the bundle metadata right?

Is this for downstream use by Porter?

[vdice]

Do we need the YAML? The spec only really supports JSON for the bundle metadata right?

It's true, an argument can be made that yaml tags shouldn't be in (similar w/ mapstructure) b/c cnab-go itself doesn't (un)marshal yaml. However, indeed, there is one current use of a third-party (Porter) benefiting from having yaml tags, as it enables us to easily unmarshal from yaml definitions into these canonical golang structs.

Definitely open to suggestions on other routes to achieve the same goal of third-party consumers wishing to re-use these canonical golang structs but may be unmarshaling from representations other than JSON.

[jeremyrickard]

I'm fine with adding the YAML, it makes things more flexible for downstream consumers.

[silvin-lubecki]

I'm fine with adding YAML too, LGTM !

[jeremyrickard]

LGTM

[glyn]

If we ever need to revisit the decision to support YAML tags, we might want to try converting from YAML to JSON and then reusing the JSON tags for demarshalling as in this article.