feat(ffe): integrate datadog-ffe (#15231)

Reopening #15098 as it was accidentally merged too early.

## Description

<!-- Provide an overview of the change and motivation for the change -->
Integrate datadog-ffe from libdatadog:
DataDog/libdatadog#1289

## Testing

<!-- Describe your testing strategy or note what tests are included -->

Tested manually that native bindings work:
```
>>> from ddtrace.internal.native._native import ffe
>>> with open("/Users/oleksii.shmalko/dd/libdatadog/datadog-ffe/tests/data/flags-v1.json", 'rb') as f:
...     json = f.read()
... 
>>> config = ffe.Configuration(json)
>>> details = config.resolve_value("numeric_flag", ffe.FlagType.Float, {})
>>> details.variant
'pi'
>>> details.value
3.1415926
```

More testing will be done as part of FFE feature.

## Risks

<!-- Note any risks associated with this change, or "None" if no risks
-->

## Additional Notes

<!-- Any other information that would be helpful for reviewers -->

This PR is stacked against
#15219

---------

Co-authored-by: Alberto Vara <alberto.vara@datadoghq.com>

Author: dd-oleksii

.riot/requirements/104618a.txt

@@ -15,7 +15,7 @@
 from ._native import SerializationError  # noqa: F401
 from ._native import TraceExporter  # noqa: F401
 from ._native import TraceExporterBuilder  # noqa: F401
-from ._native import ffande_process_config  # noqa: F401
+from ._native import ffe  # noqa: F401
 from ._native import logger  # noqa: F401
 from ._native import store_metadata  # noqa: F401
 

.riot/requirements/1346280.txt

@@ -1,4 +1,5 @@
-from typing import Dict, List, Literal, Optional
+from typing import Dict, List, Literal, Optional, Any
+from enum import Enum
 
 class DDSketch:
     def __init__(self): ...
@@ -447,11 +448,58 @@ class SerializationError(Exception):
 
     ...
 
-def ffande_process_config(config_bytes: bytes) -> Optional[bool]:
+class ffe:
     """
-    Process feature flagging and experimentation configuration rules.
-
-    :param config_bytes: Raw bytes containing the configuration data
-    :return: True if processing was successful, False otherwise, None on error
+    Native Feature Flags and Experimentation module.
     """
-    ...
+
+    class FlagType(Enum):
+        String = ...
+        Integer = ...
+        Float = ...
+        Boolean = ...
+        Object = ...
+
+    class Reason(Enum):
+        Static = ...
+        Default = ...
+        TargetingMatch = ...
+        Split = ...
+        Cached = ...
+        Disabled = ...
+        Unknown = ...
+        Stale = ...
+        Error = ...
+
+    class ErrorCode(Enum):
+        TypeMismatch = ...
+        ParseError = ...
+        FlagNotFound = ...
+        TargetingKeyMissing = ...
+        InvalidContext = ...
+        ProviderNotReady = ...
+        General = ...
+
+    class ResolutionDetails:
+        @property
+        def value(self) -> Optional[Any]: ...
+        @property
+        def error_code(self) -> Optional[ffe.ErrorCode]: ...
+        @property
+        def error_message(self) -> Optional[str]: ...
+        @property
+        def reason(self) -> Optional[ffe.Reason]: ...
+        @property
+        def variant(self) -> Optional[str]: ...
+        @property
+        def allocation_key(self) -> Optional[str]: ...
+        @property
+        def flag_metadata(self) -> dict[str, str]: ...
+        @property
+        def do_log(self) -> bool: ...
+        @property
+        def extra_logging(self) -> Optional[dict[str, str]]: ...
+
+    class Configuration:
+        def __init__(self, config_bytes: bytes) -> None: ...
+        def resolve_value(self, flag_key: str, expected_type: ffe.FlagType, context: dict) -> ffe.ResolutionDetails: ...

.riot/requirements/183bf88.txt

@@ -1,15 +1,17 @@
-from typing import Mapping
+from typing import Optional
 
+from ddtrace.internal.native._native import ffe
 
-FFE_CONFIG: Mapping = {}
+
+FFE_CONFIG: Optional[ffe.Configuration] = None
 
 
 def _get_ffe_config():
-    """Retrieve the current IAST context identifier from the ContextVar."""
+    """Retrieve the current FFE configuration."""
     return FFE_CONFIG
 
 
-def _set_ffe_config(data):
+def _set_ffe_config(config):
+    """Set the FFE configuration."""
     global FFE_CONFIG
-    """Retrieve the current IAST context identifier from the ContextVar."""
-    FFE_CONFIG = data
+    FFE_CONFIG = config