Add built-in systemd notify support - 5.7

[joaomarcos96]

Description

As discussed in #3011, this PR is a port to 5.x and because it is a new feature, a 5.7 version will be released.

This PR aims to remove the need to install the sd_notify gem by vendoring its code, thus, making the support for systemd notify built-in.

I have considered what @mperham have done with Sidekiq and said about sd_notify #2438 (comment):

With Sidekiq I decided to vendor the code for the sd_notify gem because it's only ~50 lines of stable code. I didn't want to require the user to add the gem to their gemfile manually, that shouldn't be something they need to care about IMO.

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature. PS.: I haven't added tests as the sd_notify gem code was copied exactly as it is.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

[joaomarcos96]

Could someone help me with the failing tests? I don't think they're related to the changes of this PR.

[dentarg]

Looks like tests needs to be limited to rack 2, there's Rack 3 now but only Puma 6 was updated to support that

[nateberkopec]

Closing as stale, please feel free to reopen.