structured config #2: legacy file structure refactoring and cookie support

[tuunit]

Description

Warning

This is a PR stacked on top of introduce mapstructure decoder for yaml parsing #2628

Actual stacked changes can be viewed here:
https://github.com/tuunit/oauth2-proxy/pull/6/files

1. Restructured the legacy_options.go by splitting it apart into one legacy_*.go file per type.
2. Adding support for cookie options in alpha config
3. Fixed bugs with the server config conversion and loading (missing json tags)
4. Fixed bug with unmarshaling duration types

Motivation and Context

1. Better readability and maintainability
2. Easier to debug and remove later on

How Has This Been Tested?

1. Test cases have been extend / adapted.
2. Starting the application with toml config file
3. Using the conversion function from toml to yaml
4. Starting the application with yaml config file

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.
• I have written tests for my code changes.