Add support for Strimzi-native Configurable RSA Keys

[kyguy]

Type of change

• Enhancement / new feature

Description

Adds a keySize field to CertificateAuthority allowing users to configure the RSA key size for both root and leaf certificates, defaulting to 4096 bits. This addresses compliance requirements (BSI, eIDAS, NIST) that mandate RSA keys larger than the previously hardcoded 2048-bit leaf key size.

Implements: https://github.com/strimzi/proposals/blob/main/153-configurable-ca-key-size.md

Addresses: #12769

Checklist

• Update documentation
• Update CHANGELOG.md (if present)
• Reference relevant issue(s) and close them after merging
• Write tests
• Make sure all tests pass
• Try your changes inside a Kubernetes cluster, not just from unit tests
• AI assistance was used to create this PR (see the Strimzi AI policy)

[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

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