[Dynamic Form] Fix for 1788/1794 and add styles property for support customizing styling

[wuxiaojun514]

Q	A
Bug fix?	[x]
New feature?	[x]
New sample?	[ ]
Related issues?	fixes #1788 , partially #1794

What's in this Pull Request?

This PR is mainly for

• Dynamic Form has quite a lot of problems when custom list formatting is enabled. I fixed the issues ([Dynamic Form] Cannot read properties of undefined (reading 'sections') #1788 ,[Dynamic Form] Implement @class extensibility for DynamicForm and DynamicField, add onPreRender delegate. #1794 Bad UI) .
  The author of [Dynamic Form] Implement @class extensibility for DynamicForm and DynamicField, add onPreRender delegate. #1794 provided some other approach to customize the styling but I feel this PR should work.
  Now it will show 3 columns per row if it has custom form layout
  

• Added two properties (styles and className) to support customization on styling and converted this component as style-able component. Deleted the /dynamicForm/DynamicForm.module.scss file and put its content into /dynamicForm/DynamicForm.sytles.ts and /dynamicForm/dynamicField/DynamicField.styles.ts . I kept the same classname on styling to make this change smoothly.

I have documented the new feature on DynamicForm.md and given an example how to overwrite styling

I know it is quite a big change, please let me know if anything missing.

Guidance

• You can delete this section when you are submitting the pull request.
• Please update this PR information accordingly. We'll use this as part of our release notes in monthly communications.
• Please target your PR to dev branch.

[wuxiaojun514]

Hi @joelfmrodrigues , could you look into it if you have some time?
This PR is for fixing two bugs (#1788 ,#1794 ) on Dynamic Form when list has enabled section layout and introduced className and styles property on this control so developer can customize the styling on it.
I also updated the related documentation for the new feature.
This PR changed quite a lot of code so it is very easy to have conflict with other submission ( I already fixed the conflicts two times), so I hope it can be reviewed early...
Please contact me if anything unclear or I need modify it.