Support command events

[DeanPDX]

Relevant Typescript implementation here:

https://github.com/datastax/astra-db-ts/blob/650d0f9c042e24416c33ecdb5832623d377c0deb/src/documents/events.ts#L91

Event types:

• CommandStartedEvent - Emitted when a command is started, before the initial HTTP request is made.
• CommandSucceededEvent - Emitted when a command has succeeded.
• CommandFailedEvent - Emitted when a command has errored. Probably redundant in the go world since we always force consumers to deal with errors (or intentionally ignore them).
• CommandWarningsEvent - Event emitted when the Data API returns a warning. Right now this is supported at the client/db/table/collection/cursor level. See Handle warnings #2.