feat(EAP): Trace Item resolvers (#6732)

### Context
In order to make room for new kinds of TraceItems (e.g. uptime monitors,
errors, logs, etc),add a layer of indirection to the RPC layer


Before:

```
RPC Flask Endpoint -> RPCEndpoint.execute() -> Snuba query pipeline
```

After:


```

                                           /-> EAP resolver -> Snuba query pipeline (or whatever else)
RPC Flask Endpoint -> RPCEndpoint.execute() -> Uptime resolver -> Snuba query pipeline (or whatever else)
                                           \-> Errors resolver -> Snuba query pipeline (or whatever else)
```

For rationale, please read [this
doc](https://www.notion.so/sentry/Where-does-RPC-Entity-specific-code-live-1238b10e4b5d806bb328d37c5f3bf522)

### How it works:

Every EAP endpoint has a `get_resolver` method based on the
`TraceItemName` passed in the request meta. A resolver has the same
inputs and outputs as the RPCEndpoint.

For each endpoint in EAP, there is a resolver class:

https://github.com/getsentry/snuba/pull/6732/files#diff-c018ef30d919d555c5e69908ce74e5f9144d613de3588a48d849f8c757b58628R22-R53

For each TraceItem we have, we implement the appropriate resolvers, this
PR moves EndpointTraceItemTable and EndpointTimeSeries implementations
for EAP spans into resolver implementations.

Resolvers are picked up automatically if they follow the directory
structure:

```
snuba/web/rpc/v1/resolvers
  |->R_eap_spans
  |->R_uptime
  |->R_logs
  |->R_etc
```

Implementing resolver classes in these directories will ❇️ just work ❇️
without needing to change endpoint code.

### Outstanding things

It's not clear *yet* what is really "common" and what is not. We will
figure this out through the course of doing this work

---------

Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>

snuba/web/rpc/__init__.py

@@ -5,6 +5,7 @@
 from google.protobuf.message import DecodeError
 from google.protobuf.message import Message as ProtobufMessage
 from sentry_protos.snuba.v1.error_pb2 import Error as ErrorProto
+from sentry_protos.snuba.v1.request_common_pb2 import TraceItemName
 
 from snuba import environment
 from snuba.utils.metrics.backends.abstract import MetricsBackend
@@ -25,6 +26,42 @@
 Tout = TypeVar("Tout", bound=ProtobufMessage)
 
 
+class TraceItemDataResolver(Generic[Tin, Tout], metaclass=RegisteredClass):
+    def __init__(
+        self, timer: Timer | None = None, metrics_backend: MetricsBackend | None = None
+    ) -> None:
+        self._timer = timer or Timer("endpoint_timing")
+        self._metrics_backend = metrics_backend or environment.metrics
+
+    @classmethod
+    def config_key(cls) -> str:
+        return f"{cls.endpoint_name()}__{cls.trace_item_name()}"
+
+    @classmethod
+    def endpoint_name(cls) -> str:
+        if cls.__name__ == "TraceItemDataResolver":
+            return cls.__name__
+        raise NotImplementedError
+
+    @classmethod
+    def trace_item_name(cls) -> TraceItemName.ValueType:
+        return TraceItemName.TRACE_ITEM_NAME_UNSPECIFIED
+
+    @classmethod
+    def get_from_trace_item_name(
+        cls, trace_item_name: TraceItemName.ValueType
+    ) -> "Type[TraceItemDataResolver[Tin, Tout]]":
+        return cast(
+            Type["TraceItemDataResolver[Tin, Tout]"],
+            getattr(cls, "_registry").get_class_from_name(
+                f"{cls.endpoint_name()}__{trace_item_name}"
+            ),
+        )
+
+    def resolve(self, in_msg: Tin) -> Tout:
+        raise NotImplementedError
+
+
 class RPCEndpoint(Generic[Tin, Tout], metaclass=RegisteredClass):
     def __init__(self, metrics_backend: MetricsBackend | None = None) -> None:
         self._timer = Timer("endpoint_timing")
@@ -46,6 +83,11 @@ def version(cls) -> str:
     def config_key(cls) -> str:
         return f"{cls.__name__}__{cls.version()}"
 
+    def get_resolver(
+        self, trace_item_name: TraceItemName.ValueType
+    ) -> TraceItemDataResolver[Tin, Tout]:
+        raise NotImplementedError
+
     @property
     def metrics(self) -> MetricsWrapper:
         return MetricsWrapper(