Fix multiple object redefinitions in extensions

[PierreSedon]

Currently if multiple extensions redefine the same object by adding attributes, only the modifications from the last extension to be loaded will actually be taken into account.
As an example, the core schema extends the process object for the linux extension. If someone was to also extend process in the windows extension by adding for instance a test attribute (by including a profile or just adding the attribute), this test attribute would not show in any of the APIs nor be visible through the frontend.

With the proposed fix, in the case where multiple extensions extend the same object and add the same attribute, only the last loaded extension will be taken into account which seems fine since in this case it is the schema that is poorly defined and not something the ocsf-server could really handle more gracefully.

I also included a fix for the cases where the observable_type_id_map is non-existent, if it makes more sense to put it in a dedicated pull request I can move it.

[rmouritzen-splunk]

Both commits identify real issues. The first commit looks good. For the second I noticed a less complex variation.