Private submodule support for `GitRepository` #18950

Job	Run time
docker, python:3.9	4m 3s
docker, python:3.10	5m 3s
Server Tests - python:3.9, postgres:14	2m 11s
docker, python:3.11	4m 43s
Server Tests - python:3.10, postgres:14	2m 23s
typesafety	2m 4s
docker, python:3.12	4m 35s
Server Tests - python:3.11, postgres:14	2m 19s
Server Tests - python:3.12, postgres:14	2m 17s
Server Tests - python:3.9, sqlite	2m 14s
Server Tests - python:3.10, sqlite	2m 20s
Server Tests - python:3.11, sqlite	2m 15s
Server Tests - python:3.12, sqlite	2m 13s
Client Tests - python:3.9, postgres:14	2m 21s
Client Tests - python:3.10, postgres:14	1s
Client Tests - python:3.11, postgres:14	3m 29s
Client Tests - python:3.12, postgres:14	2m 22s
Runner and Worker Tests - python:3.9, postgres:14	2m 17s
Runner and Worker Tests - python:3.10, postgres:14	2m 6s
Runner and Worker Tests - python:3.11, postgres:14	2m 17s
Runner and Worker Tests - python:3.12, postgres:14	2m 11s
combine-coverage	1s
	55m 45s

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