[nextjs][react][vue][angular] Add support for layout service REST configuration name

[orioltf]

• This PR follows the Contribution Guide
• Changelog updated

Description / Motivation

This PR adds support to define what Layout Service configuration name should be used for REST calls to Layout Service in Next.js, React, Vue and Angular packages:

• It adds a new CONFIGURATION_NAME environment variable, which by default holds the value default
• It consumes the new variable in the config-generator and adds a new property in the returned configuration object
• It consumes the new configurationName config property in the layout-service-factory to configure the RestLayoutService

REST calls to layout service can have multiple configurations in Sitecore. The linked documentation also exposes a configuration named jss.

However, the RestLayoutService has hard-coded that the configuration name has to be default, which contradicts (IMHO) the provided example in the documentation.

Testing Details

Prerequisits

• A deployed Sitecore instance with:
  • a headless topology
  • a custom LayoutService configuration named jss (for instance)
• A JSS project with Next.js flavour with:
  • proper configuration to be able to connect to the above mentioned Sitecore instance
  • DEBUG environment variable set to sitecore-jss:*
  • run jss start:connected

Test case

• Load the reported URL where Next.js can be reached in the browser (http://localhost:3000 by default)
• Check the call that Next.js makes to the layout service: it's path ends with /default.

When the default layout service configuration is not configured, miss configured, or configured in a special dedicated way, it may not return the sitecore.route object in the response, which causes the default assumptions from the routing in JSS frontend to fail and therefor to error.

• Unit Test Added
• Manual Test/Other (Please elaborate)
  Tested manually as reported in the Testing Details section

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[illiakovalenko]

@orioltf Thank you for your contribution! Our team appreciates that!
For your information, for simplicity, I pulled changes from your PR, created another one #1543, and added some minor changes and improvements. So, I close this PR