[Fleet] Update agent policy saved objects to ensure Fleet idempotency

[kpollich]

When an agent policy is created through preconfiguration, we need to ensure a unique ID is generated to guarantee idempotency in multi-Kibana environments. Currently, our default policies do not require an ID to be specified in preconfiguration, so we need to make a change that guarantees a unique ID is generated when preconfiguration creates those policies.

• When creating default agent policies during preconfiguration, generate a seeded uuidv5 id from a hard-coded string value
  • For the default Fleet policy: default-policy
  • For the default Fleet Server policy: default-fleet-server-policy

All other agent policies already require a unique ID in kibana.yml when preconfigured, so we shouldn't need to worry about idempotency there.