Add concurency configuration option for keycloak backend

[AndrienkoAleksandr]

Hey, I just made a Pull Request!

What does this pr fix:

janus-idp/backstage-plugins#2471
https://issues.redhat.com/browse/RHIDP-4793

Demo:

https://youtu.be/miQ9DgaOYcg

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Missing Changesets

The following package(s) are changed by this PR but do not have a changeset:

• @backstage-community/plugin-catalog-backend-module-keycloak

See CONTRIBUTING.md for more information about how to add changesets.

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-catalog-backend-module-keycloak	workspaces/keycloak/plugins/catalog-backend-module-keycloak	none	v3.2.4

[dzemanov]

@AndrienkoAleksandr Can you please generate changeset when you will have time?

[dzemanov]

Tested with 1000 groups and 500000 users, works as expected.
Interestingly, each scheduled run more users are commited, e.g:

Committed 27173 Keycloak users and 1000 Keycloak groups
...
Committed 31119 Keycloak users and 1000 Keycloak groups
...
Committed 42271 Keycloak users and 1000 Keycloak groups
...
Committed 43885 Keycloak users and 1000 Keycloak groups
...
Committed 44735 Keycloak users and 1000 Keycloak groups
...

So it will take more scheduled runs to commit all users in Keycloak.

Tested also locally as dynamic plugin in backstage-showcase.