importer-msgraph-metadata: Add workarounds for several bugs in Graph Api Spec

[philband]

This PR fixes some bugs I encountered whilst using the Graph SDK generated by pandora.

Model microsoft.graph.connectedOrganization

The field identitySources cannot be read-only, as it needs to be set when creating a new connected organization.
Docs Ref: https://learn.microsoft.com/en-us/graph/api/entitlementmanagement-post-connectedorganizations?view=graph-rest-beta&tabs=http#request

Model microsoft.graph.crossTenantAccessPolicyConfigurationPartner

The field tenantId cannot be read-only, otherwise it is impossible to create a new cross tenant access policy partner organization.
Docs Ref: https://learn.microsoft.com/en-us/graph/api/crosstenantaccesspolicy-post-partners?view=graph-rest-1.0&tabs=http#request

Model microsoft.graph.unifiedRoleManagementPolicy

The fields lastModifiedBy and lastModifiedDateTime must be read only, as they can only be modified by the backend service. Attempting to set them results in HTTP Error 400 Bad Request.
Docs Ref: https://learn.microsoft.com/en-us/graph/api/unifiedrolemanagementpolicy-update?view=graph-rest-1.0&tabs=http#request