Add creation for service accounts on selfhosted instances

[Djcd]

Hi,

as an addition to https://github.com/jdalrymple/gitbeaker/issues/3437

Description

The already added service accounts creation for groups is mentioned in the gitlab documentation for creating service accounts on gitlab.com (also works for selfhosted). For self hosted instances there is also another endpoint available for adding service accounts directly without a group needed. This would be my preferred way and therfore i created this pr.

This feature has also been added in GitLab 16.1.

Proposal

I created a new resource ServiceAccounts with a method for creating those accounts directly in self hosted instances.

Checklist

• [x] I have checked that this is not a duplicate issue.
• [x] I have read the documentation.

[jdalrymple]

:rocket: PR was released in 39.24.0 :rocket: