Allow MQTT config values to be entered in Captive Portal

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

As far as I know, it is not possible to enter MQTT configuration values (broker ip, username, password) without loading up an ESPHome Dashboard to add:
```
mqtt:
broker: !secret mqtt_broker
username: !secret mqtt_username
password: !secret mqtt_password
```
This does not feel all that user-friendly for someone who is abstracted away from ESPHome. I think it would be appealing to follow the pattern that exists for captive_portal/wifi. If the configuration has `captive_portal` and `wifi` setup like this:
```
captive_portal:

wifi:
ap: {}
```
It seems to create an access point for enter wifi ssid and password.

I would propose that, if `captive_portal`, `wifi, and `mqtt` are present, like this:
```
captive_portal:

wifi:
ap: {}

mqtt:
```
That the mqtt broker ip, username, and password would be able to be entered in the captive portal also.

Another option would be to have a expandable dropdown in the Captive Portal for mqtt configuration and it can be optionally entered; which would then follow the pattern of whatever the `captive_portal`/`wifi` currently does to store the ssid/password.

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

I want to flash ESPHome onto a device and give it to someone who wants to use MQTT, but has no ESPHome knowledge. I don't think there is an alternative, other than telling them to use the ESPHome dashboard; but they may be non-technical.

**Additional context**