Preserve subdomain in DNS Rewrite (e.g. [xyz].lan to [xyz].fritz.box)

### Prerequisites

- [x] Discussed and suggested to file as feature request: https://github.com/AdguardTeam/AdGuardHome/discussions/4532
- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Problem Description
Many of us have routers which have non-configurable LAN domain names. e.g. `fritz.box`

We still use the router's DHCP, because it's helpful with router features (like setting up separate guest/trusted IP ranges, firewall rules, etc). Let's say we don't want to switch to AGH DHCP just for changing the lan domain name.

Can we rewrite `*.lan` to `*.fritz.box` using AGH, i.e. rewrite the TLD preserving subdomain? It would help all LAN devices internally talk via `.lan` domain, but the address resolution can still be done by the upstream router as `.fritz.box`.

### Proposed Solution
A way to "Preserve Subdomain" when doing DNS rewrite in the AdguardHome.

Some options are:

1. Add a special parameter in `$dnsrewrite` filter rule, e.g.
```
||home.lan^$dnsrewrite=[PRESERVE_SUBDOMAIN|TLD_ONLY];CNAME;fritz.box
```
2. (OR) Add regex replace support in `$dnsrewrite` filter rule, e.g.
```
||/(.+).home.lan/^$dnsrewrite=NOERROR;CNAME;$1.fritz.box
```
3. (OR) Add a checkbox like "Preserve Wildcard" in AGH "DNS Rewrites" screen

4. (OR) Add regex replace support in AGH "DNS Rewrites" screen

### Alternatives Considered
1. It's possible to add unbind or some other DNS resolver on top of AGH, but it's quite a bit of added complexity given that AGH already has good DNS Rewrite support (screen & filter).
2. To have AGH act as a DHCP server and use AGH's lan domain by default. But as explained above, there are good reasons to use the router's DHCP.