fix: pubsub ack must rely on protocol.IsAck

[chapurlatn]

PUB/SUB Message acking is missing protocol.Result support.
For example, stan seems to properly handle it: https://github.com/cloudevents/sdk-go/blob/main/protocol/stan/v2/message.go#L66.

This PR fixes the issue with related tests.

Old cases supported:

Function result	Acked/Nacked	Finish result
nil	ACKED	nil
any error	NACKED	the original error

New behaviour:

Function result	Acked/Nacked	Finish result
nil	ACKED	nil
any error except cloudevents.Result	NACKED	the original error
cloudevents.Result (param acked)	ACKED	nil
cloudevents.Result (param nacked)	NACKED	the protocol.Result

[chapurlatn]

Note: I've found an unclosed issue about that so I mentionned the PR is there: #834

[embano1]

Can you please also take a look at action item (2) here and update the doc string?

#834 (comment)

[chapurlatn]

Hi @embano1,
The fork has been rebased from cloudevents/sdk-go main branch.
I'm on vacation during the next 2 weeks. I'll take a look anyway if you ask for changes.
Given previous comments, all has been taken into account apart the error propagation refactoring (but as I've suggested in comments, It may/should be handled in a separated PR).

[embano1]

Just a minor nit on the comment and +1 on keeping the existing Finish error handling behavior.

[duglin]

@chapurlatn wanna rebase and address Michael's comment?

[chapurlatn]

@embano1 @duglin Rebased, comments fixed as suggested, CI is green. Seems good to me now.

[duglin]

LGTM
@embano1 for the final review/merge

[embano1]

Thank you!