## Summary

This change makes certificate generation parameters configurable without changing existing defaults:

- `ebics.key.length` (default: `2048`)
- `ebics.cert.validity.years` (default: `10`)

## Motivation

Different banks and compliance profiles may require different key sizes and certificate validity windows.
Today this behavior is fixed in code.

## Changes

- Read key length from `System.getProperty("ebics.key.length")`
- Read certificate validity years from `System.getProperty("ebics.cert.validity.years")`
- Keep backward-compatible defaults when values are absent/invalid
- Add tests for both overrides in `CertificateManagerTest`

## Backward compatibility

- No behavior change for existing users unless properties are explicitly set.

## Validation

- `mvn -B test` passes locally