Formalize @ suffix for all operators (#1225)

* Formalize @ suffix for all operators

* Separate the section

* Address suggestions

httpie/cli/constants.py

@@ -15,6 +15,7 @@ SEPARATOR_HEADER = ':'
 SEPARATOR_HEADER_EMPTY = ';'
 SEPARATOR_CREDENTIALS = ':'
 SEPARATOR_PROXY = ':'
+SEPARATOR_HEADER_EMBED = ':@'
 SEPARATOR_DATA_STRING = '='
 SEPARATOR_DATA_RAW_JSON = ':='
 SEPARATOR_FILE_UPLOAD = '@'
@@ -41,6 +42,7 @@ SEPARATORS_GROUP_MULTIPART = frozenset({
 
 # Separators for items whose value is a filename to be embedded
 SEPARATOR_GROUP_DATA_EMBED_ITEMS = frozenset({
+    SEPARATOR_HEADER_EMBED,
     SEPARATOR_QUERY_EMBED_FILE,
     SEPARATOR_DATA_EMBED_FILE_CONTENTS,
     SEPARATOR_DATA_EMBED_RAW_JSON_FILE,
@@ -56,6 +58,7 @@ SEPARATOR_GROUP_NESTED_JSON_ITEMS = frozenset([
 SEPARATOR_GROUP_ALL_ITEMS = frozenset({
     SEPARATOR_HEADER,
     SEPARATOR_HEADER_EMPTY,
+    SEPARATOR_HEADER_EMBED,
     SEPARATOR_QUERY_PARAM,
     SEPARATOR_QUERY_EMBED_FILE,
     SEPARATOR_DATA_STRING,

httpie/cli/requestitems.py

@@ -8,7 +8,8 @@ from .constants import (
     SEPARATOR_DATA_EMBED_RAW_JSON_FILE, SEPARATOR_GROUP_NESTED_JSON_ITEMS,
     SEPARATOR_DATA_RAW_JSON, SEPARATOR_DATA_STRING, SEPARATOR_FILE_UPLOAD,
     SEPARATOR_FILE_UPLOAD_TYPE, SEPARATOR_HEADER, SEPARATOR_HEADER_EMPTY,
-    SEPARATOR_QUERY_PARAM, SEPARATOR_QUERY_EMBED_FILE, RequestType
+    SEPARATOR_HEADER_EMBED, SEPARATOR_QUERY_PARAM,
+    SEPARATOR_QUERY_EMBED_FILE, RequestType
 )
 from .dicts import (
     BaseMultiDict, MultipartRequestDataDict, RequestDataDict,
@@ -48,6 +49,10 @@ class RequestItems:
                 process_empty_header_arg,
                 instance.headers,
             ),
+            SEPARATOR_HEADER_EMBED: (
+                process_embed_header_arg,
+                instance.headers,
+            ),
             SEPARATOR_QUERY_PARAM: (
                 process_query_param_arg,
                 instance.params,
@@ -119,6 +124,10 @@ def process_header_arg(arg: KeyValueArg) -> Optional[str]:
     return arg.value or None
 
 
+def process_embed_header_arg(arg: KeyValueArg) -> str:
+    return load_text_file(arg).rstrip('\n')
+
+
 def process_empty_header_arg(arg: KeyValueArg) -> str:
     if not arg.value:
         return arg.value