Document SSL/TLS key and certificate formats accepted by plugins

[edmocosta]

Plugins are not being specific about the accepted SSL key/certificate format. The manticore-based plugins, for example, are not documenting that the SSL keys should be in the PCK8 format, otherwise it won't work.

A few examples where the documentation is not clear about that:

• https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-ssl_key
• https://www.elastic.co/guide/en/logstash/current/plugins-filters-elasticsearch.html#plugins-filters-elasticsearch-ssl_key
• https://www.elastic.co/guide/en/logstash/current/configuring-centralized-pipelines.html (xpack.management.elasticsearch.ssl.key)

The goal of issue is to review all plugins, and ensure that the ones supporting SSL/TLS configurations, have the key and certificate format specified on the docs.

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### Tasks
- [x] Elasticsearch output (https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/1181)
- [ ] ...