feat!: ability to select namespace from composite field path

[tenstad]

Description of your changes

Related to https://crossplane.slack.com/archives/C08BBMDCH7W/p1769948003900819

Reworks the current namespace: string to configure either a Value or to be an extracted value FromCompositeFieldPath.

namespace:
  type: FromCompositeFieldPath
  valueFromFieldPath: metadata.namespace

namespace:
  type: Value
  value: crossplane-system

The selector struct for namespace reuses the selector used for labels values today, making it easy to extend both to e.g. support FromEnvironmentFieldPath in the future.

Could do the same non-breakingly by adding a new namespaceFrom field instead of modifying the existing namespace.

See commit log for step by step implementation process.

Fixes #61

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[tenstad]

Closing as breaking changes are not desired. Could recreate under a different key if desired.