Make some new task manager variables configurable via kibana.yml

[mikecote]

As part of #188194, we're introducing new capabilities into the Task Manager. A few of those capabilities should be customizable via kibana.yml in case we need to make adjustments for a given deployment/project.

• DISCOVERY_INTERVAL

  kibana/x-pack/plugins/task_manager/server/kibana_discovery_service/kibana_discovery_service.ts

  Line 24 in 1d5a0e1

  export const DISCOVERY_INTERVAL = 1000 * 10;

• ACTIVE_NODES_LOOK_BACK https://github.com/elastic/kibana/blob/1d5a0e1a44ae410b24b438af3fa9eb317d79d4e2/x-pack/plugins/task_manager/server/kibana_discovery_service/kibana_discovery_service.ts#L25C14-L25C36

• KIBANAS_PER_PARTITION

  kibana/x-pack/plugins/task_manager/server/lib/assign_pod_partitions.ts

  Line 8 in 1d5a0e1

  const KIBANAS_PER_PARTITION = 2;

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)