fix: Defer to user-supplied input before disabling execute-api endpoint

[dgholz]

Description

Allow users to leave the default endpoint enabled when a custom domain is set.

Motivation and Context

Setting the disable_execute_api_endpoint variable to false should leave the default endpoint enabled when a custom domain is set.
This change resolves #128
This change also preserves the existing behaviour of disabling the default endpoint when a custom domain is set & no value was given for the disable_execute_api_endpoint variable.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have used this change in my own Terraform module to see that the default endpoint was not disabled
• I have executed pre-commit run -a on my pull request

[dgholz]

I'm happy to add an example to cover this behaviour, but would appreciate some guidance on where to put it & what to include.