Support configuring a TLS key passphrase in APM Integration

The APM integration currently does not support configuring a TLS key passphrase. The main reasons where that the passphrase would have been sent and stored in plain text on the Elastic Agent host. https://github.com/elastic/elastic-agent/pull/398 introduced improvements where agent configuration is now encrypted. We can make use of this and add support for a TLS key passphrase.