add schemas to support attaching an exact array of vestings to any issuances with vesting (as an option to replace the need for vesting_terms_id)

[jacobyavis]

What type of PR is this?

/kind feature

What this PR does / why we need it:

To allow an implementer of OCF to forego the VestingTerms schema in favour of a simpler "manual" style schedule (a simple list of dates and amounts)

Which issue(s) this PR fixes:

Fixes #509

[JSv4]

Thanks for putting this together @jacobyavis