Add deletion policy field and decision logic - #32

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@jaypipes @tomba7 @zainawaisn

1. What should happen on delete when spec.deletionPolicy is not set?
   I'm thinking if the resource was created by the operator, delete and if it was adopted retain.

2. Before sending the delete request, Decide checks for delete protection and Nexus endpoints itself, even though Temporal Cloud already rejects the delete in both cases. Should I keep those checks, or just send the delete and surface Cloud's error? Checking first gives clearer messages and avoids repeated rejected deletes.
   Relying on Cloud means less duplicated logic but its error codes overlap (in both cases temporal returns FailedPrecondition), so we'd have to show its message as is.

What should happen on delete when spec.deletionPolicy is not set?
I'm thinking if the resource was created by the operator, delete and if it was adopted retain.

IMO we should definitely set a default. My hunch is that most users who are familiar with ACK/Crossplane will expect Delete, since that's the default in those tools. But apart from the familiarity, either option is fine with me. I'd say we go with Delete by default (regardless of how it was created) and then adjust it once we have customer feedback or when we get to the resource adoption implementation.

Before sending the delete request, Decide checks for delete protection and Nexus endpoints itself, even though Temporal Cloud already rejects the delete in both cases. Should I keep those checks, or just send the delete and surface Cloud's error? Checking first gives clearer messages and avoids repeated rejected deletes.
Relying on Cloud means less duplicated logic but its error codes overlap (in both cases temporal returns FailedPrecondition), so we'd have to show its message as is.

On the surface, the delete protection check seems like an obvious one to keep but unsure about the Nexus endpoint one since I haven't reviewed the PR just yet (will do shortly). In general, my preference is to keep the operator's own checks to a minimum, make tcloud the source of truth, and only add operator side checks when it's obvious or necessary (e..g the namespace CRD only has basic validation and leaves the rest to tcloud)