[FS#3356] https-dns-proxy: Luci interface breaks the configuration if a custom server is used

[OpenWrt Bugs]

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A new Flyspray task has been opened.  Details are below. 

User who did this - Damian Nowak (Nowaker) 

Attached to Project - OpenWrt/LEDE Project
Summary - https-dns-proxy: Luci interface breaks the configuration if a custom server is used
Task Type - Bug Report
Category - Packages
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - openwrt-19.07
Due in Version - Undecided
Due Date - Undecided
Details - When you edit `/etc/config/https-dns-proxy` and set a custom server, not known to Luci HTML interface, the Luci UI at /cgi-bin/luci/admin/services/https-dns-proxy will display "CIRA Canadian Shield (Family)" as the selected resolver. This, by itself is not a problem yet, as the Proxy will function as expected.

However, the moment one makes any changes in the UI, e.g. changing listen port, `/etc/config/https-dns-proxy` will get rewritten and an actual resolver for CIRA Canadian Shield (Family) will be used.

Luci interface for HTTPS DNS Proxy should:

1. Bare minimum: indicate a custom resolver is used, and not lose the resolver after making changes to listen port and other stuff.
1. Nice to have: Allow a user to define a custom resolver in the UI - by specifying `resolver_url`, `bootstrap_dns`, `user` and `group` properties


The problem applies on any device as it's not device-specific.


Reproduction instruction:

1. Edit `/etc/config/https-dns-proxy` and make it look like this:

```
config main 'config'
        option update_dnsmasq_config '-'

config https-dns-proxy
        option listen_addr '127.0.0.1'
        option listen_port '5054'
        option user 'nobody'
        option group 'nogroup'
        option bootstrap_dns '1.1.1.1,1.0.0.1,2606:4700:4700::1111,2606:4700:4700::1001'
        option resolver_url 'https://1.1.1.1/dns-query'
```
2. Go to http://192.168.10.1/cgi-bin/luci/admin/services/https-dns-proxy and observe "CIRA Canadian Shield (Family)" as the selected resolver.
3. Change listen port and click Save & Apply.
4. Observe `/etc/config/https-dns-proxy` lose `resolver_url` setting.

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3356

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