Support git:: (and gitlab::) submodules

[gaborcsardi]

It uses authentication for the submodules if credentials are available in the git credential store.

Does not work currently for github:: packages sources. We can support those as well, if needed. Then we'll have to extract the downloaded repo snapshot.

Closes #354.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.14493% with 55 lines in your changes are missing coverage. Please review.

Project coverage is 86.32%. Comparing base (b704a10) to head (d4434e1).

❗ Current head d4434e1 differs from pull request most recent head 3343d13. Consider uploading reports for the commit 3343d13 to get more accurate results

Files	Patch %	Lines
R/git-submodules.R	53.04%	54 Missing ⚠️
R/type-git.R	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #358      +/-   ##
==========================================
- Coverage   86.76%   86.32%   -0.45%     
==========================================
  Files          62       63       +1     
  Lines        7127     7252     +125     
==========================================
+ Hits         6184     6260      +76     
- Misses        943      992      +49     

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