@spec prepare_headers(
  AntikytheraAws.Auth.Credentials.t(),
  String.t(),
  String.t(),
  Antikythera.Http.Method.t(),
  Antikythera.UnencodedPath.t(),
  binary(),
  Antikythera.Http.Headers.t(),
  [{String.t(), String.t()}]
) :: Antikythera.Http.Headers.t()

From given creds and request informations, returns headers ready for AWS request.

Arguments are mostly Antikythera.Httpc.request/5 compatible, with some extra treatments:

• creds, region, service must be supplied
  • creds are basically retrieved with AntikytheraAws.Auth.InstanceProfileCredentials
• path must not include query string. Relative/redundant path must be resolved beforehand
  • Any characters in segments except RFC3986 "Unreserved Characters", will be encoded
  • Escape chars ("%") in segments will be encoded to "%25"
• payload serialization must be done beforehand, since it will be hashed in signature and compared on AWS
• headers must have "host" field, which is a hard-requirement for signature
  • "x-amz-date" or "date" field value will be used as signature date. "x-amz-date" will be prioritized
  • If neither exist, "x-amz-date" will be generated with current date
• params must be decoded query strings, in a list of tuple-2
  • To convert from Antikythera.Http.QueryParams (which is Croma.SubtypeOfMap), just use Map.to_list/1