Allow users to set custom global banners in the UI

#### Feature Description
In the system where there are multiple environments that are separate Consul clusters users may need a way to determine which environment they are looking at.

Relevant change has been already done to Nomad and is being prepared for Vault. However there doesn't seem similar feature available for Consul (apart of running your own copy of patched UI).

Nomad issue: https://github.com/hashicorp/nomad/issues/10813
Vault issue: https://github.com/hashicorp/vault/issues/6584

Ideally would be a solution similar to Nomad to be able to set text, text color and background color like such:

```json
{
"ui": {
"enabled": "true",
"nomad": {
"ui_url": "https://nomad_host:4646/ui"
},
"label": {
"text": "Production",
"background_color": "red",
"text_color": "#000000"
}
}
}
```

#### Use Case(s)

This is helpful for UI users whom support multiple environments to easily distinguish between them.

Start by reviewing the linked Nomad issue and the comparable Vault issue, then inspect how Consul's UI configuration is exposed. Define how the requested label text, text color, and background color would reach the UI; done means users can configure and see distinct environment banners in the Consul UI.

Written by the indexing model from the issue text.