generic fetcher: Add models for lockfile

Adds pydantic models that will be used to represent and validate
the lockfile for the generic fetcher.

Signed-off-by: Jan Koscielniak <[email protected]>

cachi2/core/package_managers/generic/models.py

@@ -0,0 +1,37 @@
+from typing import Literal, Optional
+
+from pydantic import BaseModel, field_validator
+
+
+class LockfileMetadata(BaseModel):
+    """Defines format of the metadata section in the lockfile."""
+
+    version: Literal["1.0"]
+
+
+class LockfileArtifact(BaseModel):
+    """Defines format of a single artifact in the lockfile."""
+
+    download_url: str
+    target: Optional[str] = None
+    checksums: dict[str, str]
+
+    @field_validator("checksums")
+    @classmethod
+    def no_empty_checksums(cls, value: dict[str, str]) -> dict[str, str]:
+        """
+        Validate that at least one checksum is present for an artifact.
+
+        :param value: the checksums dict to validate
+        :return: the validated checksum dict
+        """
+        if len(value) == 0:
+            raise ValueError("At least one checksum must be provided.")
+        return value
+
+
+class GenericLockfileV1(BaseModel):
+    """Defines format of the cachi2 generic lockfile, version 1.0."""
+
+    metadata: LockfileMetadata
+    artifacts: list[LockfileArtifact]