Resolves #1216.

* Add support for spoiler formatting

* Implement support for sending message attachments that are spoilers

* use consistent naming

* update docstring for new isSpoiler param

* move extension method to be under core project, make spoiler prefix a const

* typo fix

* update missing xmldocs

* move SpoilerPrefix const outside of interface

* Add isSpoiler support to webhook client

adds the isSpoiler field to uploading files with a webhook, which will only
insert "SPOILER_" to the start of the filename. This does not include other
fields in the payload, as this is not in the documentation, and was not observed
like in the regular client