Update all commands to rely solely on callbacks

[nealrosen]

Resolves issues #23, #51, and #52

Changes in this PR include

• Removed direct return values from all commands
• Added callback support for all commands
• Updated addEventHandler command to return initial EventListener response through callback instead of return value
• Added success parameter to all callbacks
• Fixed 2 typos in the sample stub code
• Removed getGPPData command

[lamrowena]

@nealrosen can you please also update the description of applicableSections to the following -- applicableSections: Array of Number, // Section ID considered to be in force for this transaction. In most cases, this field should have a single section ID. In rare occasions where such a single section ID can not be determined, the field may contain up to 2 values. During the transition period which ends on July 1, 2023, the legacy USPrivacy section may be determined as applicable along with another US section. In this case, the field may contain up to 3 values where one of the values is 6, representing the legacy USPrivacy section. The value can be 0 or a Section ID specified by the Publisher / Advertiser, during stub / load. When no section is applicable, the value will be -1.

This addresses Issue #21.

[nealrosen]

@lamrowena I have updated the applicableSections description. I see there are now conflict between the PR and the develop branch. At least part of the conflict is due to a recent commit made by HeinzBaumann. I think we should review the conflicts on today's call to determine how to resolve them.