feat: Allow bypassing init-local stage if not needed

[rajibade]

Proposed Commit Message

Allow the init-local stage to be skipped for configurations that do not need it.

Some images configure a list with network-only datasources. For these images, the init-local stage
just occupies the critical path and doesn't configure anything. Since moving to the single process
optimisation, the socket sync makes skipping it difficult. This adds a check for an opt-in marker at 
`/etc/cloud/cloud-init-local.disabled` just for disabling this stage. Default behaviour unchanged.

Additional Context

This is primarily done testing with amazon linux on EC2, since it uses ec2-net-utils in its network
management but I think it has general use too. Happy to hear alternatives or revisions.

Test Steps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)