Add release automation which publishes release binaries

[chasinglogic]

This adds a continuous delivery pipeline which will run on release tags. It currently will build a release binary for Linux x86_64, Linux ARM, and MacOS x86_64 (which can be run on M* series macs).

It has code in it already to support cross compiling using cross which can be setup later. I've yet to get it working flawlessly on my own projects so I have left the setup code in but not utilised it.

This fixes https://github.com/altsem/gitu/issues/50

[chasinglogic]

You'll need to setup a github token and add it to the project secrets for this to work. You can find instructions for that here and here:

• https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens#creating-a-fine-grained-personal-access-token
• https://docs.github.com/en/actions/security-guides/using-secrets-in-github-actions#creating-secrets-for-a-repository

[altsem]

You'll need to setup a github token and add it to the project secrets for this to work. You can find instructions for that here and here:

* https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens#creating-a-fine-grained-personal-access-token

* https://docs.github.com/en/actions/security-guides/using-secrets-in-github-actions#creating-secrets-for-a-repository

Actually GITHUB_TOKEN is set automatically, I think it already has access to the release api. Guess it's easy to find out by trying.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.03%. Comparing base (ecd068f) to head (70f3e99). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #52 +/- ## ========================================== + Coverage 82.68% 83.03% +0.34% ========================================== Files 32 32 Lines 2484 2564 +80 ========================================== + Hits 2054 2129 +75 - Misses 430 435 +5 ```

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