feat: Update stock plan to support multiple stock class ids

[lavens]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This PR adds support for multiple stock classes to be associated with a stock plan.

Which issue(s) this PR fixes:
Fixes #461

cc @jarrettq

[jarrettq]

Our approach placed an emphasis on not introducing breaking changes, so the following items are worth pointing out:

1. We've opted to mark fields like stock_class_id in the Stock Plan object as deprecated, with a goal to eventually sunset it in favor of a stock_class_ids list.
2. stock_class_id is still an optional field in the Equity Compensation Issuance transaction, but we'd recommend eventually transitioning it to a required field to handle scenarios where a Stock Plan starts off with a single share class but adds support for other share classes in subsequent updates to it.