Trailers and in-memory certs

Added

• Add support for using in-memory client certificates. (#89, #320)
• Add API for accessing trailer headers in responses. (#157, #256)

Changed

• Isahc's MSRV has been increased from 1.41.0 to 1.46.0. (#321)

Fixed

• Fix wrong link to request type in docs. (#323) @humb1t

Dependency Updates

• Update Public Suffix List to 598c638 (#325) @teto-bot