feat: [CI-18308]: Add Cosign Image Signing Support

[Ompragash]

Adds container image signing capability using Cosign directly within the drone-docker plugin. Now supports signing images using their immutable digests with proper fallback to tag-based signing.

New Inputs:

• PLUGIN_COSIGN_PRIVATE_KEY: Private key content (PEM) or file path
• PLUGIN_COSIGN_PASSWORD: Optional password for encrypted keys
• PLUGIN_COSIGN_PARAMS: Optional additional cosign parameters

Features:

• Signs using immutable digest references (repo@sha256:digest) for better security
• Falls back to tag-based signing if digest unavailable
• Non-interactive operation with COSIGN_YES=true
• Handles both embedded PEM keys and file paths
• Signs after push completes, ensuring registry digest is used

Testing:

cosign-drone-docker.mp4