Removing accessToken defaults from schema to prevent leaks (#351)

### Summary:
- This will prevent manually created provider resource from saving
EnvVar value into state file of the stack
- The default provider resource can still potentially leak accessToken
value, if it is specified in the config as plaintext (highly
non-advisable)

### Testing:
- Integ tests
- Manually tests

CHANGELOG_PENDING.md

@@ -4,6 +4,7 @@
 - Fixed import by refactoring Read method of AccessToken resource + minor refactor [#311](https://github.com/pulumi/pulumi-pulumiservice/issues/311)
 - Fixed import by refactoring Read method of AgentPool resource + minor refactor [#311](https://github.com/pulumi/pulumi-pulumiservice/issues/311)
 - Fixing noisy diff in DS OIDC object [#330](https://github.com/pulumi/pulumi-pulumiservice/issues/330)
+- Removed accessToken provider parameter defaults from schema to prevent leaks [#350](https://github.com/pulumi/pulumi-pulumiservice/issues/350)
 
 ### Miscellaneous
 - Added CHANGELOG_PENDING file to ignore-list of the `main` workflow [[#340](https://github.com/pulumi/pulumi-pulumiservice/issues/340)]

provider/cmd/pulumi-resource-pulumiservice/schema.json

@@ -25,12 +25,7 @@
       "accessToken": {
         "description": "Access Token to authenticate with Pulumi Cloud.",
         "type": "string",
-        "default": "",
-        "defaultInfo": {
-          "environment": [
-            "PULUMI_ACCESS_TOKEN"
-          ]
-        }
+        "secret": true
       }
     }
   },