feat: locally run gitaggregate

[theangryangel]

Description

Initial quick lunch time hack to test out git-autoshare. This is ripped out of my dev copy and not suitable for general consumption yet.

It does 3 things:

• It allows invoke git-aggregate to optionally run on the host/developer device, rather than in a container, in a backwards-compatible way - defaults to current behaviour to allow developers to opt-in
  • When running locally it defaults the number of jobs to the number of cores on the device - this will probably need to be pinned to a maximum of X to avoid disk issues on devices with A Large Number of Cores(TM)
• It allows pre-commit install to be skipped (this is very rarely needed, but is useful for speed)

As it is so far it shaves off the start up time of the container, which

Pros:

• More easily opens up git-autoshare, or similar tools, to more easily to save diskspace and bandwidth
• ssh-agent, etc. become options
• Per-project keys could disappear, easing developer workflow
  • Question: do we want this? it would avoid the whole "generate a key, add it to GitHub as a deployment key" workflow, at the expense of CI needing to change.
• Opens up possibilities for [DRAFT] [RFD]: Does Doodba as a base still make sense for us?  #61
  • Opens up possibilities to move addon linking/pruning outside of the container (whether or not there's any point here is something to be discussed in [DRAFT] [RFD]: Does Doodba as a base still make sense for us?  #61)
• Allows to get all the source code without building or running a container (very rarely have we needed this)

Cons:

• Requires gitaggregate installed locally, which comes with all the baggage of incorrect versions/updates, etc.
  • This could be replaced by suggesting users do stuff in a devcontainer. Don't like this as it means developers can't use their preferred tools.

Associated risk level medium

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update