Generate code for AppSettings

[nickrandolph]

Current support for appsettings is for the appsettings.json (and appsettings.platform.json) to be read on application startup. This doesn't perform well, particularly on WASM where it requires waiting for the file store to be accessible.

Proposed alternative is to code generate a wrapper for appsettings which would have the values defined as application constants, meaning that no file access is required as runtime

[nickrandolph]

Assumption:

• Supports appsettings.json and appsettings.environment.json as sources
• Extensible so developers and add their own files eg mysettings.json
• Shouldn't break existing api for enabling appsettings
• Build action on files can be Content or Embedded Resource. Setting build action to None will ignore the files

Code Generation

For each settings file, a corresponding class will be created with a method that can be invoked to load the information into the IConfiguration.

Two options:

• Generated file contains a single string that represents the contents of the appsettings file
• Generated file contains more optimized code that avoids parsing the whole appsettings file

Question: How do we integrate the generated files into the HostBuilder extension methods?