Fix how host configuration is handled in WebApplicationBuilder (#36186) (#36233)

- The new WebApplicationBuilder merged host configuration and application configuration together into the same configuration source, then applied those changes to the final all as host configuration. This had some bad side effects:
  - All configuration changes are wrapped in a chained configuration source which prevented updates (that's a runtime bug)
 - Host configuration could be mutated by changing any value directly in configuration. This meant that different parts of the code base could see a different environment, application name and content root.
This change fixes things by snapshotting the immutable host configuration in the constructor and applying it later. Then only applying configuration sources to application configuration, not host configuration.
- Added tests