Add submodule support for Git stacks

[Westie]

Important

Marked as draft pending advice relating to creating Postgres migrations

Proposed change

Introduction

This PR adds in support for submodules within Git stacks.

I've created a new property for git repositories, called submodulesEnabled.

This property, if enabled, effectivelly calls the following commands upon syncing a git repository, syncing a git stack and deploying a git stack:

git submodule sync --recursive
git submodule update --init --recursive

FYI I have checked, the .git folder that belongs to the submodule is not copied across from the repository to the stack.

Since deployments of stacks don't actually take git into consideration, all we need to do is actually ensure the submodule is properly set up - which this does to the best of its ability.

Closes #917

What it looks like

Type of change

• Bug fix: non-breaking change which fixes an issue.
• New feature / Enhancement: non-breaking change which adds functionality.
• Breaking change: fix or feature that would cause existing functionality to not work as expected.
• Other. Please explain:

[CLAassistant]

All committers have signed the CLA.