add snakeoil certificates

[parimoh]

This PR is meant to address #54. I'm gonna test this / research a bit more on this as well just to make sure that it actually works as intended and does everything it's supposed to.

Summary

Adds make-ssl-cert functionality to both Debian and Alpine variants for generating self-signed certificates which allows for SSL/TLS testing capabilities.

Changes Made

Debian Images:

• Added ssl-cert package to runtime dependencies
• Uses existing openssl command-line tools (installed as dependency)
• Provides standard make-ssl-cert generate-default-snakeoil command
• Creates certificates at /etc/ssl/certs/ssl-cert-snakeoil.pem and /etc/ssl/private/ssl-cert-snakeoil.key

Alpine Images:

• Added minimal make-ssl-cert shell script
• Uses existing openssl package
• Syntax: make-ssl-cert <cert_path> <key_path>

Usage Examples

Debian:

docker run --rm valkey:debian make-ssl-cert generate-default-snakeoil

Alpine:

docker run --rm valkey:alpine make-ssl-cert /tmp/cert.pem /tmp/key.pem

Testing

• Both variants generate valid X.509 certificates with 10-year validity
• Certificate subject: CN=localhost
• Verified certificate/key pair matching
• No breaking changes to existing functionality
• OpenSSL installation confirmed on both variants

Size Impact

• Debian: +20KB (ssl-cert package)
• Alpine: +200 bytes (shell script)

[roshkhatri]

Can we make this script like entrypoint.sh and copy it to the image??
We are essentially creating it on runtime here

[roshkhatri]

Also can we add tests for these?