[googlechat] Improve OS notification display

[loganmzz]

Google Chat have some caveats regarding OS notification display when pushing messages.

• Desktop OS notification only display plain-text content (text field): https://developers.google.com/workspace/chat/api/reference/rpc/google.chat.v1#google.chat.v1.Message.FIELDS.string.google.chat.v1.Message.text
• Mobile OS notification only display fallback content (fallback_text_ field): https://developers.google.com/workspace/chat/api/reference/rpc/google.chat.v1#google.chat.v1.Message.FIELDS.string.google.chat.v1.Message.fallback_text

So, it would be nice to support:

• both text and cardV2: I think having a dedicated text field along the current cardsV2 one. AND ALSO an option to just reuse notification message as currently when no cardsV2 is provided.
• fallback_text: I think having a dedicated fallback_text (or any other case) field along the current cardV2 one. AND ALSO an option to juste reuse notification message.

[loganmzz]

I may try to provide a PR. I will take a look on tomorrow (CEST) if I can make some try.