feat: Update test harness with metadata assertions #1467 (#289)

## This PR
Adds gherkin test to verify that flag evaluations provide metadata

### Related Issues

Part of #1467
([https://github.com/open-feature/flagd/issues/1467](https://github.com/open-feature/flagd/issues/1467))

### Follow-up Tasks
Implement steps of the gherkin file in the repositories, and add test
data according to the issue

---------

Signed-off-by: christian.lutnik <christian.lutnik@dynatrace.com>
Signed-off-by: Todd Baert <todd.baert@dynatrace.com>
Co-authored-by: Todd Baert <todd.baert@dynatrace.com>
Co-authored-by: Simon Schrottner <simon.schrottner@dynatrace.com>

specification/assets/gherkin/metadata.feature

@@ -0,0 +1,27 @@
+@Metadata
+Feature: Metadata
+
+  Background:
+    Given a stable provider
+
+  Scenario: Returns metadata
+    Given a Boolean-flag with key "metadata-flag" and a default value "true"
+    When the flag was evaluated with details
+    Then the resolved metadata should contain
+      | key     | metadata_type | value |
+      | string  | String        | 1.0.2 |
+      | integer | Integer       | 2     |
+      | float   | Float         | 0.1   |
+      | boolean | Boolean       | true  |
+
+  Scenario Outline: Returns no metadata
+    Given a <flag_type>-flag with key "<key>" and a default value "<default_value>"
+    When the flag was evaluated with details
+    Then the resolved metadata is empty
+
+    Examples: Flags
+      | key          | flag_type | default_value |
+      | boolean-flag | Boolean   | true          |
+      | integer-flag | Integer   | 23            |
+      | float-flag   | Float     | 2.3           |
+      | string-flag  | String    | value         |