feat(extension): add pglinter

[pmpetit]

This pull request introduces the pglinter PostgreSQL extension for CloudNativePG, providing all necessary packaging, documentation, and metadata to support its deployment and management. The main changes include adding a multi-stage Dockerfile for building a CloudNativePG-compliant extension image, usage documentation, license information, and integration metadata.

Fixes : #69

CloudNativePG extension packaging:

• Added a multi-stage Dockerfile (pglinter/Dockerfile) that extracts the pglinter extension from its .deb package and builds a minimal, CloudNativePG-compatible container image. This ensures the extension can be deployed as a sidecar image according to CloudNativePG standards.

Documentation and metadata:

• Added a README.md (pglinter/README.md) with instructions for deploying and enabling the pglinter extension in a CloudNativePG cluster, including example manifests and verification steps.
• Added a metadata.hcl (pglinter/metadata.hcl) file describing the extension’s metadata, supported versions, and integration points for the CloudNativePG ecosystem.

Licensing:

• Added a copyright file (pglinter/copyright) with licensing information for the extension.