feat: add chain and fullchain certificate file support

[krakenhavoc]

Summary

• Add chain-path and fullchain-path inputs (default ./chain.pem and ./fullchain.pem)
• Add chain-path and fullchain-path outputs with absolute paths
• Issue, download, and renew commands now produce all three cert files (leaf, chain, fullchain)
• Set 0644 permissions on chain and fullchain files

Test plan

• Run action with command: issue and verify chain.pem and fullchain.pem are created alongside cert.pem
• Run action with command: download and verify all three files are downloaded
• Run action with command: renew and verify chain files are refreshed after renewal
• Depends on: feat: add full certificate chain download and display support cli#7 (CLI chain flags), feat: add full certificate chain support app#79 (backend chain endpoint)