The api module previously had defaultPartition and defaultNamespace vars
for when we need default/empty split usage between ent/oss respectively.
This commit moves those two variables out of test code so that they can
be used for the service exports config entry's `GetNamespace()` method.
Previously `GetNamespace()` would return "default" in both OSS and enterprise,
which can trip up automation that passes the result of this method as the
namespace to write a config entry.
The split vars are kept private to prevent external usage, and prefixed with
`split` for more clarity about their behavior.
Fixes#8755
Since I was updating the interface, i also added the missing `GetNamespace()`.
Depending upon how you look at it, this is a breaking change since it adds methods to the exported interface `api.ConfigEntry`. Given that you cannot define your own config entry kinds, and all of the machinery of the `api.Client` acts like a factory to construct the canned ones from the rest of the module, this feels like it's not a problematic change as it would only break someone who had reimplemented the `ConfigEntry` interface themselves for no apparent utility?
