mysql-backup init at 1.0.0-rc3

[Ramblurr]

Description of changes

This PR introduces a new package mysql-backup and a nixos module services.databacker-mysql-backup to configure it.

A few notes:

1. This PR is in draft mode until an upstream PR is merged that patches mysql-backup to enable it to connect over the local unix domain socket.
2. I am submitting the PR now in hopes to start a review of the NixOS module.
3. Once that PR is merged and a new release is made I will update the package (the module does not need updating) to point to upstream (not my fork)
4. I've included extensive tests for the module

Module name conflict

There already exists a services.mysqlBackup module that is a small collection of bash scripts. This PR does not have anything to do with that module.

But then why add this module if services.mysqlBackup already exists?

I think this comparison table speaks for itself:

Feature	services.mysqlBackup	services.databacker-mysql-backup (this PR)
Backup local databases by name	x	x
Backup to a local directory	x	x
Prune and rotate backups according to a retention schedule		x
Backup all local databases without enumeration		x
Backup remote databases		x
Backup to S3-compat endpoint		x
Backup to Samba share		x

Also, I am not happy with the name services.databacker-mysql-backup but I thought that services.mysql-backup, while available was too close to the existing services.mysqlBackup. I'm open to better name suggestions, but please no bike shedding.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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