Copy ChatMessage meta fields in ChatPromptBuilder

[vblagoje]

Describe the bug
We don't copy meta fields when we render ChatMessage with template

To Reproduce

1. Create a ChatMessage
2. Set its content to some jinja template
3. Set meta field of that message to some value
4. Pass the message through ChatPromptBuilder
5. Witness how message got rendered but its meta field is lost

Describe your environment (please complete the following information):

• OS: OSX
• Haystack version: 2.4
• Integration version:

Note

Discovered when experimenting with AnthropicChatGenerator in #1006