Add support for snaps

[damonbarry]

This change updates the repo to support building IoT Edge as a snap package. One of the main differences with snaps is that they run in a tightly confined sandbox, so many hard-coded references to users and filesystem paths had to be adapted to support compile-time variables.

I confirmed the CI Build pipeline passes and produces snaps. The end-to-end tests were adapted to support snaps, and I've confirmed they pass. I also ensured that CE Test, ISA95 Smoke Tests, Nested end-to-end tests, and Connectivity tests all pass against these bits (although snaps are not integrated into these pipelines).

Azure IoT Edge PR checklist:

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines and Best Practices

• [X] I have read the contribution guidelines.
• [X] Title of the pull request is clear and informative.
• [X] Description of the pull request includes a concise summary of the enhancement or bug fix.

Testing Guidelines

• [X] Pull request includes test coverage for the included changes.
• Description of the pull request includes
  • [X] concise summary of tests added/modified
  • [X] local testing done.

[vadim-kovalyov]

@damonbarry, are you going to merge this eventually? I have #7230 that is blocked by this (needs new required env vars to compile). So this PR should go in first.

If this is not going to be merged, then I can just do mio version bump without i-i-s ref update.

[damonbarry]

@damonbarry, are you going to merge this eventually? I have #7230 that is blocked by this (needs new required env vars to compile). So this PR should go in first.

If this is not going to be merged, then I can just do mio version bump without i-i-s ref update.

@vadim-kovalyov Ugh, you're right. :) I am working on getting this merged, but it's not going to happen today. I'm ok with whichever route you choose...

[vadim-kovalyov]

@damonbarry, are you going to merge this eventually? I have #7230 that is blocked by this (needs new required env vars to compile). So this PR should go in first. If this is not going to be merged, then I can just do mio version bump without i-i-s ref update.

@vadim-kovalyov Ugh, you're right. :) I am working on getting this merged, but it's not going to happen today. I'm ok with whichever route you choose...

No rush, I'll follow up next week.