fix: remove comparable constraint from Optional (#6041)

* fix: remove comparable constraint from Optional

This allows making slice types (`[]any`) optional, which is permitted at
the API level, but wasn't allowed at the type constraint level.

We previously used this constraint because it was assumed that a user
might constraint an Optional manually, and not set `isSet`. However,
this is an edge case that pokes into the private internals of
`dagger.gen.go`, and we now have the `Opt` helper for this. Users doing
this kind of access should be aware of what they're doing.

Signed-off-by: Justin Chadwell <[email protected]>

* feat: add OptEmpty helper to Go SDK

This creates an empty optional - note that the type constraint cannot be
automatically derived, at least in the current version of Go.

Signed-off-by: Justin Chadwell <[email protected]>

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Signed-off-by: Justin Chadwell <[email protected]>