Add an external identifier field to the user entity

Problem/Motivation

We need to implement `externalId` for user entity type. More info why this is needed here => https://github.com/goalgorilla/open_social/pull/3942

Steps to reproduce

- install field_ui
- As a admin, go to /admin/config/people/accounts/fields
- Confirm that you can see field_external_indentifier field and that it is locked (see screenshot).

Proposed resolution

Add an external identifier field to the user entity. This field is locked, as it should not be managed
via the UI, but rather by modifying the configuration. The field is provided without any settings and is, therefore, unusable by default. At least one target type needs to be specified by modifying the field
storage configuration to ensure the field can be
properly used.

**Field usage example:**
SCIM provides the concept of externalId which
can be used to identify a resource with an identifier that is known to the SCIM client. This prevents the SCIM client from having to store a mapping between its internal data and our ID, instead putting that burden on us as a service provider.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A