feat: protected environments

[Vadims-U]

Description of your changes

This PR adds support for configuring GitLab Protected Environments via provider-gitlab.
It implements the Protected Environments API and introduces a new managed resource that allows users to define deploy access levels, approval rules, and required approval counts for project environments.

Fixes #269

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I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

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How has this code been tested

Automated

Added unit tests for client-side delta calculation and reconciliation logic.
Added controller unit tests covering Observe / Create / Update / Delete behavior and error paths.
Verified idempotent reconciliation behavior locally.
Ran make reviewable test

Manual

Tested locally against GitLab API using a standard Crossplane development setup.
Applied and updated ProtectedEnvironment resources and verified:

deploy access level changes
approval rules and required approval count handling
correct late initialization behavior

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Test environment

Windows 11 with WSL2 (Ubuntu)
Podman‑based local Kubernetes cluster
Crossplane installed via Helm
Provider built and run locally in debug mode

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Example manifest used during testing

apiVersion: projects.gitlab.m.crossplane.io/v1alpha1
kind: ProtectedEnvironment
metadata:
  name: prod-protected
  namespace: default
spec:
  forProvider:
    projectId: "12345678"
    name: "production"

    deployAccessLevels:
      - accessLevel: 60 # Owner

    approvalRules:
      - groupId: 123456789
        requiredApprovals: 2

  providerConfigRef:
    kind: ProviderConfig
    name: gitlab-provider