Install-DbaFirstResponderKit: Added ability to create certificates to let public execute FRK procedures

[ReeceGoding]

Be warned, this may be terrible! Specifically:

• It has been too many years since I've had to seriously think about certificates. Claude will surely outsmart me here.
• I am unsure what problem the CREATE USER part of the original code was trying to solve.
• I had to do a lot of this in T-SQL. I know of no PowerShell interface for certificates.
• The error handling probably isn't good enough, but I don't know certificates or our standards well enough to make it much better.

Type of Change

• Bug fix (non-breaking change, fixes # )
• New feature (non-breaking change, adds functionality, fixes Install-DbaFirstResponderKit: add recommended certificates for non-DBAs #10368)
• Breaking change (affects multiple commands or functionality, fixes # )
• Ran manual Pester test and has passed (Invoke-ManualPester)
• Adding code coverage to existing functionality
• Pester test is included
• If new file reference added for test, has is been added to github.com/dataplat/appveyor-lab ?
• Unit test is included
• Documentation
• Build system

Purpose

Added ability to create certificates to let public execute FRK procedures. Closes #10368

Also added sp_kill to existing ValidateSet.

Approach

It started as an attempt to copy the "How to Grant Permissions to Non-DBAs" section here, but that set of steps assumes that you are in master. To make other databases also work, I had to expand that with the tricks mentioned here.

Commands to test

Install-DbaFirstResponderKit and specifically, the new -LetPublicExecute parameter.

Screenshots

Learning

1. While running the integration tests locally, I found that Invoke-ManualPester -Path ./tests/Install-DbaFirstResponderKit.Tests.ps1 -ScriptAnalyzer -TestIntegration fails if you don't have a C:\Temp folder. If you know what you're doing, then you can add a new $config['temp'] = "/path/i/actually/have" line to dbatools/tests/constants.local.ps1, but there is nothing anywhere to hint that you need to do that. Everything else I've ever wanted to change in dbatools/tests/constants.local.ps1 already had a placeholder.
2. Trying to test this with EXECUTE AS USER = 'guest' was a very bad idea. It seems unable to take the permissions from the certificate.
3. Get-DbaDbUser does not see certificate-mapped users?