Allow Customization of Captive Portal UI

**Describe the problem you have/What new integration you would like**

It would be great if you we could supply a custom stylesheet for the captive portal. This would allow customization of the entire UI, adding a logo using data URIs and such.

Currently the captive portal feature doesn't have any configuration, so adding optional config to specify either CSS inline, or point to a file, would be great. There may need to be limits on the size of the CSS.

Ideally this would look like:

```yaml
captive_portal:
custom_styles: {string}
```

If the value starts with a `/` then it is interpreted as a file path, if it's anything else, it would be interpreted as a stylesheet. Either way, it should be injected into the page using `…` tags.

**Please describe your use case for this integration and alternatives you've tried:**

The use case is to allow makers of "Made for ESPHome" devices to customize their users onboarding experience. This ties nicely into the new ESPHome onboarding experience in Home Assistant.

I don't believe there are any viable alternatives for this except a local esphome change.

**Additional context**

None

The issue names no files, tests, or entry points; first locate the captive portal UI and its configuration handling. Confirm the request is still wanted, then define how optional inline and file-based custom_styles values, size limits, and stylesheet injection should work; done means the portal renders the requested customization.

Written by the indexing model from the issue text.