[Fleet] 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.

-- edit

Some more details

hostsPath = "/api/fleet/fleet_server_hosts"
hostPath = "/api/fleet/fleet_server_hosts/{itemId}"

// This is an 8.6.2 regression, it actually can be nullable however.
// It is also present in the POST/PUT output request bodies.
// [request body.proxy_id]: definition for this key is missing
hostsPath Post "requestBody.content.application/json.schema.properties.proxy_id"
hostPath  Put "requestBody.content.application/json.schema.properties.proxy_id"

For the PUT output request body, ID is present resulting in the following error:
// strict_dynamic_mapping_exception: [1:345] mapping set to strict, dynamic introduction of [id] within [ingest-outputs] is not allowed"
"schemas.update_output_%s.properties.id"

Package policy POST/PUT request body have output_id as nullable, see the following error:
// [request body.0.output_id]: expected value of type [string] but got [null]
// [request body.1.output_id]: definition for this key is missing"
"schemas.package_policy_request.properties.output_id"

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

cc @juliaElastic - might be something to look at in the new router implementation.