Add Trello receiver

[dougbu]

• first part of #181

Supporting changes

Add WebHookVerifySignatureFilter methods needed for Trello receiver

• FromBase64(...) and its GetBase64DecodedHash(...) wrapper
• GetRequestBodyHash_SHA1(...) overloads taking prefix and suffix parameters
• GetRequestBodyHash_SHA256(...) overload taking a suffix parameter

Support short-circuiting HTTP HEAD requests

• add IWebHookGetHeadRequestMetadata.AllowHeadRequests
• IWebHookGetRequestMetadata -> IWebHookGetHeadRequestMetadata
• WebHookGetRequestFilter -> WebHookGetHeadRequestFilter

nit: add missing <item> in Order comments of a few filters (bad merge?)

[dougbu]

🆙📅 Also rebased on dougbu/in.the.small (#232)

[dougbu]

:up::date: to clean up From*(...) methods @rynowak please have a quick look, focusing on the last commit

[dougbu]

• d00d67fa13, support short-circuiting HEAD requests
• 547c5202b0, improve WebHookVerifySignatureFilter API
  • includes new methods for Trello receiver
• 9b6c58751e, add Trello receiver