Add simple authenticator

... including some required authenticator infrastructure

PFERD/authenticators/__init__.py

@@ -0,0 +1,19 @@
+from configparser import SectionProxy
+from typing import Callable, Dict
+
+from ..authenticator import Authenticator
+from ..conductor import TerminalConductor
+from ..config import Config
+from .simple import SimpleAuthenticator, SimpleAuthSection
+
+AuthConstructor = Callable[[
+    str,                # Name (without the "auth:" prefix)
+    SectionProxy,       # Authenticator's section of global config
+    Config,             # Global config
+    TerminalConductor,  # Global conductor instance
+], Authenticator]
+
+AUTHENTICATORS: Dict[str, AuthConstructor] = {
+    "simple": lambda n, s, c, t:
+        SimpleAuthenticator(n, SimpleAuthSection(s), c, t),
+}

PFERD/authenticators/simple.py

@@ -0,0 +1,48 @@
+from typing import Optional, Tuple
+
+from ..authenticator import Authenticator, AuthSection
+from ..conductor import TerminalConductor
+from ..config import Config
+from ..utils import agetpass, ainput
+
+
+class SimpleAuthSection(AuthSection):
+    def username(self) -> Optional[str]:
+        return self.s.get("username")
+
+    def password(self) -> Optional[str]:
+        return self.s.get("password")
+
+
+class SimpleAuthenticator(Authenticator):
+    def __init__(
+            self,
+            name: str,
+            section: SimpleAuthSection,
+            config: Config,
+            conductor: TerminalConductor,
+    ) -> None:
+        super().__init__(name, section, config, conductor)
+
+        self.username = section.username()
+        self.password = section.password()
+
+        self.username_fixed = self.username is not None
+        self.password_fixed = self.password is not None
+
+    async def credentials(self) -> Tuple[str, str]:
+        if self.username is not None and self.password is not None:
+            return self.username, self.password
+
+        async with self.conductor.exclusive_output():
+            if self.username is None:
+                self.username = await ainput("Username: ")
+            else:
+                print(f"Username: {self.username}")
+
+            if self.password is None:
+                self.password = await agetpass("Password: ")
+            else:
+                print("Password: *******")
+
+        return self.username, self.password