feat: Automatic attribute updates for form-associated proxy

[robarbms]

Description

Motivation & context

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[ghost]

All CLA requirements met.

[robarbms]

Changes are up and running on my server.

Testing was a little tricky. If you click on something in the frame, it will draw focus, but I found it easiest to break the frame out using the 'Open canvas in new tab' button in the upper right corner. - http://robdor:6006/iframe.html?id=checkbox--base

Then you can run this script in the developer console:
["checkbox","slider","radio","switch","text-area","button","text-field","option"].reduce((t,c)=>t.concat(Array.from(document.getElementsByTagName(fast-${c}))),[]).forEach(e=>{console.log(["placeholder","formEnctype","formAction","formMethod","formNoValidate","formTarget","required","disabled","type","name","readOnly","min","max","step","autofocus","list"].reduce((o,a)=>{if(e.proxy[a]!==undefined&&e.proxy[a]!==null&&e.proxy[a]!==""){o[a]=e.proxy[a]}return o},{}));e.onchange=(i)=>setTimeout(()=>console.log(["checked","value","selected"].reduce((o,c)=>{o[c]=i.target.proxy[c];return o},{})),10)})

[robarbms]

I will work on updating attributes through behaviors.