Add automated Maven Central publishing to release workflow

[jeremyprime]

Summary

Adds a publish job to the release workflow that signs and deploys artifacts to Maven Central (staging) after the existing build/test/release steps complete.

Closes #85

Changes

• New publish job gated by maven-publish environment approval
• Uses OIDC to assume AWS IAM role (no static credentials in GitHub)
• Fetches Maven and GPG credentials from AWS Secrets Manager
• Configures GPG for non-interactive signing
• Deploys spring-data-valkey and spring-boot-starter-data-valkey to Sonatype staging

Prerequisites

• AWS IAM role with OIDC trust policy
• Secrets in AWS Secrets Manager for Maven credentials and GPG key/passphrase
• GitHub environment with required reviewers and AWS_ROLE_ARN variable