It is not possible to zero OAuth2.0 credentials via the fleet UI

When the oauth secret has been added, it cannot be removed via the UI. This breaks some uses. The work-around is to edit the POST text to remove those and then force the change.

Steps:
1. add a cel custom integration changing only the auth; remove the Basic Authentication user and password and add the OAuth2 authentication fields.
```
POST kbn:/api/fleet/package_policies
{
"policy_id": "",
"package": {
"name": "cel",
"version": "1.12.0"
},
"name": "cel-1",
"description": "",
"namespace": "",
"inputs": {
"cel-cel": {
"enabled": true,
"streams": {
"cel.cel": {
"enabled": true,
"vars": {
"data_stream.dataset": "cel.cel",
"resource_url": "https://server.example.com:8089/api",
"resource_interval": "1m",
"program": "# // Fetch the agent's public IP every minute and note when the last request was made.\n# // It does not use the Resource URL configuration value.\n# bytes(get(\"https://api.ipify.org/?format=json\").Body).as(body, {\n# \"events\": [body.decode_json().with({\n# \"last_requested_at\": has(state.cursor) && has(state.cursor.last_requested_at) ?\n# state.cursor.last_requested_at\n# :\n# now\n# })],\n# \"cursor\": {\"last_requested_at\": now}\n# })\n",
"regexp": "#products: '(?i)(Elasticsearch|Beats|Logstash|Kibana)'\n#solutions: '(?i)(Search|Observability|Security)'\n",
"username": "",
"password": "",
"digest_username": "",
"digest_password": "",
"oauth_id": "abc",
"oauth_secret": "abc",
"oauth_token_url": "http://example.com/",
"redact_fields": [],
"delete_redacted_fields": false,
"resource_redirect_headers_ban_list": [],
"oauth_scopes": [],
"tags": [
"forwarded"
]
}
}
}
}
}
}
```
2. Save and continue.
3. Edit the policy to remove the OAuth2 configurations and save.
4. Examine the resulting policy.
```
PUT kbn:/api/fleet/package_policies/fa519c2a-3dc6-4133-8b25-3ec50bfa53b5
{
"package": {
"name": "cel",
"version": "1.12.0"
},
"name": "cel-1",
"namespace": "",
"description": "",
"policy_id": "elastic-agent-managed-ep",
"vars": {},
"inputs": {
"cel-cel": {
"enabled": true,
"streams": {
"cel.cel": {
"enabled": true,
"vars": {
"data_stream.dataset": "cel.cel",
"resource_url": "https://server.example.com:8089/api",
"resource_interval": "1m",
"program": "# // Fetch the agent's public IP every minute and note when the last request was made.\n# // It does not use the Resource URL configuration value.\n# bytes(get(\"https://api.ipify.org/?format=json\").Body).as(body, {\n# \"events\": [body.decode_json().with({\n# \"last_requested_at\": has(state.cursor) && has(state.cursor.last_requested_at) ?\n# state.cursor.last_requested_at\n# :\n# now\n# })],\n# \"cursor\": {\"last_requested_at\": now}\n# })\n",
"regexp": "#products: '(?i)(Elasticsearch|Beats|Logstash|Kibana)'\n#solutions: '(?i)(Search|Observability|Security)'\n",
"username": "",
"password": "",
"digest_username": "",
"digest_password": "",
"oauth_id": "",
"oauth_secret": {
"id": "Bt7kMJIBg_TidvVzgDb6",
"isSecretRef": true
},
"oauth_token_url": "",
"redact_fields": [],
"delete_redacted_fields": false,
"resource_redirect_headers_ban_list": [],
"oauth_scopes": [],
"tags": [
"forwarded"
]
}
}
}
}
}
}
```
5. Note that oauth_secret is still present. This can cause an issue for users who are developing a CEL-based custom integration, accidentally add OAuth2.0 cred and then need to back them out as the resulting configuration will not validate as correct.

The work around is to edit the POST text to remove that field. This is not an immediately obvious approach.

This looks like it may be a limitation of the handlebars template, though I'm not sure whether it could be due to how fleet handles configurations transitions to the zero state (also due to handlebars). I do not think this is limited to the CEL custom integration, but this is where I found it, and there are very few packages where removing OAuth is a valid action to take. HTTPJSON is one, for the same reasons and is also affected.