Add defaultNamespace in pkgi/app crs

[praveenrewar]

What this PR does / why we need it:

Add defaultNamespace field in App and PackageInstall spec. This can be used to set the default namespace where the resources will be created. Currently this is the same as the App namespace.

Which issue(s) this PR fixes:

Fixes #1318

Does this PR introduce a user-facing change?

Introduce defaultNamespace in Apps and PackageInstalls. This can be used to set the default namespace where the resources will be installed.

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[neil-hickey]

Looks good, just a couple of smaller things about formatting / libs used. The overall code and whatnot looks good

[cppforlife]

not sure if yall already settled on the name, if so ignore, but if not, id like to throw in defaultNamespace as an alternative name. imho reasons why it's better:

• installation feels much closer to PackageInstall but this field is also available on App
• this namespace is used as a default namespace for resources that do not specify a namespace, but doesnt actually override it afaik

[neil-hickey]

LGTM