Improve startup time with lazy loading some args (#1221)

* Improve startup time with lazy loading some args

* add some tests

* Add changelog entry

* Update CHANGELOG.md

Co-authored-by: Jakub Roztocil <jakub@roztocil.co>

httpie/cli/utils.py

@@ -0,0 +1,53 @@
+import argparse
+from typing import Any, Callable, Generic, Iterator, Iterable, Optional, TypeVar
+
+T = TypeVar('T')
+
+
+class LazyChoices(argparse.Action, Generic[T]):
+    def __init__(
+        self,
+        *args,
+        getter: Callable[[], Iterable[T]],
+        help_formatter: Optional[Callable[[T], str]] = None,
+        sort: bool = False,
+        cache: bool = True,
+        **kwargs
+    ) -> None:
+        self.getter = getter
+        self.help_formatter = help_formatter
+        self.sort = sort
+        self.cache = cache
+        self._help: Optional[str] = None
+        self._obj: Optional[Iterable[T]] = None
+        super().__init__(*args, **kwargs)
+        self.choices = self
+
+    def load(self) -> T:
+        if self._obj is None or not self.cache:
+            self._obj = self.getter()
+
+        assert self._obj is not None
+        return self._obj
+
+    @property
+    def help(self) -> str:
+        if self._help is None and self.help_formatter is not None:
+            self._help = self.help_formatter(self.load())
+        return self._help
+
+    @help.setter
+    def help(self, value: Any) -> None:
+        self._help = value
+
+    def __contains__(self, item: Any) -> bool:
+        return item in self.load()
+
+    def __iter__(self) -> Iterator[T]:
+        if self.sort:
+            return iter(sorted(self.load()))
+        else:
+            return iter(self.load())
+
+    def __call__(self, parser, namespace, values, option_string=None):
+        setattr(namespace, self.dest, values)