Add diagnostics collect command to elastic-agent. (#28461)

Add diagnostics collect command to elastic-agent.

Add diagnostics collect command to elastic-agent. This option will
create an archieve with information that can help with debugging the
elastic-agent. The information includes process metadata (same as the
output of elastic-agent diagnostics), config information, the rendered
policy, and any local log files.

(cherry picked from commit 99ebf3e)