Added internal useConfig hook [SDK-2264]

[Widcket]

Description

If the developer set up their login handler URL to be something other than the default /api/auth/login, then they would have to pass the custom value on every call to withPageAuthRequired.

This PR moves the configuration of custom URLs out of withPageAuthRequired to the UserProvider. Plus it's now also possible to set it with an environment variable that is accessible from the client-side, enabling the developer to just set it once in a single place. This results in an improved developer experience.

Also, it's possible to customize the profile handler URL via an environment variable as well, saving the developer from having to pass it to the UserProvider.

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

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