add initial version of vault extension

[HarshCasper]

Summary

• Add HashiCorp Vault extension for LocalStack using Docker sidecar pattern
• Pre-configure Vault dev mode with KV v2, Transit secrets engines, and AWS IAM auth
• Configure IAM auth to validate against LocalStack's STS endpoint for Lambda integration
• Include a sample app demonstrating Lambda + Vault secrets retrieval via Vault Lambda Extension
• Provide Makefile automation for extension installation, sample app deployment, and testing

[whummer]

Very cool, kudos for adding this one @HarshCasper ! 🚀 I just checked out the branch and ran the sample - works like a charm. Nice to see that it also uses the Vault Lambda Extension layer, which nicely gets pulled into LocalStack. 👌

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My only concern here is - hashicorp/vault is published under a BSL license, so we should first do another legal review before publishing this extension to our customers (ideally we'll get in touch with folks at HashiCorp to get them on board with this). Happy to discuss this offline. 👍