fix: Broaden definition of hook data value types. (#307)

## This PR
Loosens the value definition for hook data.

In the example within the spec, and within the implementation in the
dotnet-sdk, the intent was to allow any type of data. For example
creating an open telemetry span in before, storing it hook data, and
ending it in after.

### Notes
Related to: open-feature/dotnet-sdk#387

---------

Signed-off-by: Ryan Lamb <4955475+kinyoklion@users.noreply.github.com>

Author: kinyoklion

specification.json

@@ -928,7 +928,7 @@
         {
             "id": "Requirement 4.6.1",
             "machine_id": "requirement_4_6_1",
-            "content": "`hook data` MUST be a structure supporting the definition of arbitrary properties, with keys of type `string`, and values of type `boolean | string | number | datetime | structure`.",
+            "content": "`hook data` MUST be a structure supporting the definition of arbitrary properties, with keys of type `string`, and values of any type.",
             "RFC 2119 keyword": "MUST",
             "children": []
         },

specification/sections/04-hooks.md

@@ -330,4 +330,11 @@ but different hooks have different hook data instances.
 
 #### Requirement 4.6.1
 
-> `hook data` **MUST** be a structure supporting the definition of arbitrary properties, with keys of type `string`, and values of type `boolean | string | number | datetime | structure`.
+> `hook data` **MUST** be a structure supporting the definition of arbitrary properties, with keys of type `string`, and values of any type.
+
+Access to hook data is restricted to only a single hook instance, and it has no serialization requirements, and as a result does not require any value type restrictions.
+
+Example TypeScript definition:
+```JavaScript
+type HookData = Record<string, unknown>;
+```