API alignment: handler is always in options

[milyin]

There was an misalignment between API handler/callback parameter:

• naming can be callback (for callback only items) or handler (for items which may accept callback or channel). This is wrong: even if at this moment the callaback parameter doesn't allow chanllel., it may/should support it in the future. Therefore parameter name should be handler
• in declare_subscriber handler parameter was passed in arguments, while in other places it's passed in options. Made this uniform: now handler is always in options. (in fact this approach differs from the one in C++ which takes handler in parameters. But there is no function overloading in typescript, so approach with handler in options is better here)

[github-actions]

PR missing one of the required labels: {'internal', 'breaking-change', 'enhancement', 'documentation', 'dependencies', 'bug', 'new feature'}