feat: Make use of the main route table for the intra subnet implicit association.

[maruyama-shintaro-ppcd]

Description

make use of the main route table for the intra subnet implicit association by adding a variable var.create_intra_subnet_route_table.

Motivation and Context

I would like to use the main route table for the intra subnet implicit association. By specifying the var.create_intra_subnet_route_table with false, users can choose implicit association to the main route table for intra subnet.

Breaking Changes

There is no breaking changes.

How Has This Been Tested?

• [ ] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [x] I have tested and validated these changes using one or more of the provided examples/* projects
• [x] I have executed pre-commit run -a on my pull request

[maruyama-shintaro-ppcd]

@bryantbiggs Hello maintainer, I appreciate your hard work. Do you happened to have a time to look at this PR? If you have any questions, please let me know.