Add destination namespace check in indexer

[dtomcej]

What does this PR do?

This PR:

• Adds a check for an empty destination namespace in traffictargets during load
• Adds test to ensure empty destination namespaces are filled with the namespace of the traffictarget.

Fixes #546

Additional Notes

We may want to look at a "validate resource" method in the future, but I think for now it can be avoided as there is not a lot of validation required.

[jspdown]

@dtomcej Thanks for the PR. Initially I was thinking about doing this in the indexSMIResources. The sooner is the better I think.

[SantoDE]

Just a general question first:

I see that this takes an empty namespace as default, which I think is good. However, how do we handle the case if Source or Destination don't have a namespace set. Do we infer the Namespace of the TrafficTarget?

[dtomcej]

Good call @SantoDE. I checked the CRD, and both source and destination have namespace as an optional field, however the entire source field itself is optional.

I will assume that no destination namespace defined would result in the default namespace being used, but that could easily be changed in the future. For the time being, we will use default, so that code functions properly.

I have opened an issue: servicemeshinterface/smi-spec#162 that should hopefully answer this question.

[SantoDE]

Thanks @dtomcej.

Seeing that the issue is mostly answered by Thomas already, I can live with that. We should just document the behaviour tough :-)

[dtomcej]

As answered in the SMI spec issue above, we will have the namespace inherited from the TT object itself.