[Good First Issue] Add aegisctl federation status command

[saivedant169]

Add a 'federation status' subcommand to aegisctl that calls GET /admin/v1/federation/planes and prints data plane health in a table.

Files: cmd/aegisctl/main.go
Acceptance: Running 'aegisctl federation status --url http://localhost:8081' shows data planes with name, health, last seen, request count.