feat: add AnnotatedStringInterface (#72)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Introduced a new interface for annotated strings, allowing for flag
metadata and callable status to be associated with inputs and outputs.
This enhances flexibility when working with annotated strings in rule
definitions.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: johanneskoester

src/snakemake_interface_common/io.py

@@ -0,0 +1,18 @@
+from abc import ABC, abstractmethod
+from typing import Any, Dict
+
+
+class AnnotatedStringInterface(ABC):
+    """Interface for annotated strings/callables as they are used for input/output in
+    Snakemake rules.
+    """
+
+    @property
+    @abstractmethod
+    def flags(self) -> Dict[str, Any]: ...
+
+    @abstractmethod
+    def is_callable(self) -> bool: ...
+
+    def is_flagged(self, flag: str) -> bool:
+        return flag in self.flags and bool(self.flags[flag])