Feat selector transforms

[jtucci]

Description of your changes

Adds transform support to label selectors in EnvironmentConfig selection. This allows field path values to be transformed before being used as label match values, bringing a subset of function-patch-and-transform's transform functionality to environment config selection.

Supported transforms:

• Map — value lookup from a key-value map
• Match — pattern-based matching (literal and regexp) with fallback
• String — Format, Convert (ToUpper/ToLower), TrimPrefix, TrimSuffix, Regexp (group extract + replace with backreferences), Replace

Key changes:

• input/v1beta1/transforms.go — Transform types (subset of patch-and-transform, excluding math/convert/base64/hash/join as they don't produce valid label values)
• transforms.go — Transform resolution logic, faithfully copied from function-patch-and-transform with regexp replace backreference support added
• transforms_test.go — Unit tests for map, match, and string transforms
• fn.go — Changed GetString() to GetValue() on field path resolution to support non-string fields (booleans, numbers), with transforms applied before converting to label string
• input/v1beta1/environment.go — Added Transforms field to EnvironmentSourceSelectorLabelMatcher
• Added examples/selector_with_transforms/ demonstrating regexp group extract, regexp replace with backreferences, map transform, and case conversion

Example use case:

matchLabels:
  - key: config
    valueFromFieldPath: "metadata.labels[crossplane.io/claim-namespace]"
    transforms:
      - type: string
        string:
          type: Regexp
          regexp:
            match: "^team-(.+)$"
            replace: "${1}-environment-config"

team-alpha → alpha-environment-config

Fixes #100

I have:

• Read and followed Crossplane's [contribution process].
• Added or updated unit tests for my change.
  [contribution process]: https://git.io/fj2m9
  [docs]: https://docs.crossplane.io/contribute/contribute

[jtucci]

Hey @phisco :) Is this feature something we can add in?