json: cannot unmarshal bool into Go struct field Event.metadata of type string

Executing `event, err := client.GetProjectEvent(org, project, eventId)` returns this error:

> json: cannot unmarshal bool into Go struct field Event.metadata of type string

Event metadata appears to be defined as:
> `Metadata *map[string]string `json:"metadata,omitempty"`

Looking at the response, there can be non string data types like this boolean:

```json
"metadata": {
"display_title_with_tree_label": false
}
```

This change resolves the error:
> Metadata *map[string]interface{} `json:"metadata,omitempty"`

Start at the Event struct and the GetProjectEvent entry point, then reproduce the response containing a boolean value in metadata. Confirm that the event response unmarshals without the reported type error and that existing metadata behavior remains intact.

Written by the indexing model from the issue text.