Add an example `standalone_git` with a NiFi Registry container integrated with GIT version control

[hannemariavister]

Closes/fixes/resolves issue(s)?

Closes #20

What was added/changed/fixed?

Add an example with a NiFi Registry container integrated with GIT version control:

• Created a new directory: terraform-nomad-nifiregistry/example/standalone_git
• Added terraform-nomad-nifiregistry/nomad/nifi_registry_git_integration.hcl
• Updated terraform-nomad-nifiregistry/variables.tf and terraform-nomad-nifiregistry/main.tf with more variables for git configuration.

Checklist (after created PR)

• [x] Added reviewers
• [ ] Added assignee(s)
• [x] Added label(s)
• [ ] Added to project
• [x] Added to milestone
• Need to update documentation? (Y/n)

[AditiSharma77]

error in testing @https://github.com/hannemariavister/terraform-nomad-nifiregistry/pull/27/checks?check_run_id=1681421806

[hannemariavister]

Can you update the link for example/standalone_git in README file line 38?

Done 👍