Various OAS fleet request fields are incorrectly nullable

[daemitus]

Describe the bug:

As part of upgrading the Terraform elasticstack provider to use the generated OAS docs, various fields were discovered in fleet that are described as nullable, but are in-fact not. These were discovered by running the TF acceptance tests locally. This is probably due to re-use of the response schema as the request schema (or vice-versa, with some minimal changes).

There are likely more, these are just what the acceptance tests caught.

agentPoliciesPath = "/api/fleet/agent_policies"
agentPolicyPath = "/api/fleet/agent_policies/{agentPolicyId}"

// [request body.keep_monitoring_alive]: expected value of type [boolean] but got [null]
agentPoliciesPath Post "requestBody.content.application/json.schema.properties.keep_monitoring_alive"
agentPolicyPath    Put "requestBody.content.application/json.schema.properties.keep_monitoring_alive"

// [request body.supports_agentless]: expected value of type [boolean] but got [null]
agentPoliciesPath Post "requestBody.content.application/json.schema.properties.supports_agentless"
agentPolicyPath.   Put "requestBody.content.application/json.schema.properties.supports_agentless"

// [request body.supports_agentless]: expected value of type [boolean] but got [null]
agentPoliciesPath Post "requestBody.content.application/json.schema.properties.overrides"
agentPolicyPath   Put  "requestBody.content.application/json.schema.properties.overrides"

outputByIdPath = "/api/fleet/outputs/{outputId}"
outputsPath = "/api/fleet/outputs"
// These are generally in each output type POST/PUT request, after I created a $ref entry.
// [request body.1.ca_sha256]: expected value of type [string] but got [null]"
"schemas.new_output_<type>.properties.ca_sha256"
"schemas.update_output_<type>.properties.ca_sha256"

// [request body.1.ca_trusted_fingerprint]: expected value of type [string] but got [null]
"schemas.new_output_%s.properties.ca_trusted_fingerprint"
"schemas.update_output_%s.properties.ca_trusted_fingerprint"

// [request body.0.shipper]: expected a plain object value, but found [null] instead
"schemas.new_output_shipper"
"schemas.new_output_ssl"

// [request body.1.shipper]: expected a plain object value, but found [null] instead
"schemas.update_output_shipper"
"schemas.update_output_ssl"

Steps to reproduce:

curl -u elastic:password http://localhost:5601/api/fleet/outputs -H 'Kbn-Xsrf: true' --json '{"type": "elasticsearch", "name": "testing", "hosts": ["http://localhost:9200"], "shipper": null}'
{"statusCode":400,"error":"Bad Request","message":"[request body]: types that failed validation:\n- [request body.0.shipper]: expected a plain object value, but found [null] instead.\n- [request body.1.shipper]: expected a plain object value, but found [null] instead.\n- [request body.2.shipper]: expected a plain object value, but found [null] instead.\n- [request body.3.shipper]: expected a plain object value, but found [null] instead."}%

Expected behavior:
If truly nullable, no errors. Otherwise OAS should show optional, not nullable for each POST/PUT request.