Prepare 0.7.0 release

[djc]

• Followup from Add aws_lc_rs feature and set it as default #56 to clean things up a bit
• Cleanup from Update to hyper dependency to version 1 #50
• Restore ability to build HttpClient implementations without hyper (after Update to hyper dependency to version 1 #50)
• Incorporate Fix #51 handle unknown challenge types #54 (fixes Future proof the parsing of challenge types #51)

[valkum]

Only a small recommendation. Looks good otherwise.