Add Elastic Agent support

[Oddly]

The collection currently supports Filebeat, Metricbeat, and Auditbeat through the beats role. Elastic Agent is Elastic's unified agent that replaces individual Beats with a single binary managed by Fleet Server or standalone configuration.

Adding Elastic Agent support would let users deploy and manage Elastic Agent alongside or instead of individual Beats. This could be a new elastic_agent role or an extension of the existing beats role.

Scope to consider:

• Install and configure Elastic Agent (standalone mode)
• Fleet Server enrollment (fleet-managed mode)
• TLS certificate management (reuse existing CA infrastructure)
• Policy configuration for log, metric, and security data collection
• Migration path from individual Beats to Elastic Agent
• Support for both 8.x and 9.x

The Logstash role already has logstash_input_elastic_agent and logstash_input_elastic_agent_port variables for receiving data from Elastic Agent, so the pipeline side is partially ready.

[Oddly]

Merged into #16 which has the same scope. Continuing discussion there.