fix(admin-settings): refactor setting API to have custom save logic for setting sections

[ravinderk]

User Story

As a developer, I want to have logic to set custom save logic for setting section.
For example Email setting page. Each core email data stores in give-settings which is getting bigger. The developer can easily create a custom setting page and option name which prevents him to store data in give-settings options.
We can not do that for a section like if we define a new email setting then it is difficult to store email data in custom option and render section with saved data.
For example reminder emails in https://github.com/WordImpress/Give-Recurring-Donations/issues/640