Update the websub-code base according to code-review comments

[ayeshLK]

Description:

• According to the comments received on websub code review, update the code-base.

Describe your solution(s)

• Implement constants for websub common-responses and use them throughout the module.
• Use string-template to generate subscriber-callback-url.
• Use included-record-params for websub:ListenerConfiguration in websub:Listener init method.
• Use mime constants to check content-types in content-distribution.
• Include support for x-form-urlencoded in content-distribution.
• Mark parameters in RequestQueryParams record as optional.

Related Issues (optional):

• Support form-urlencoded content type for contnt-distribution notification #1106