Filterlist categorization as 'safe browsing' or 'parental control'

### Prerequisites

- [x] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [x] I have searched other issues and found no duplicates

- [x] I want to request a feature or enhancement and not ask a question

### The problem

AdGuard Home collects metrics about why it responded to questions with the answer(s) it did. When safe browsing or parental control is enabled, AGH denotes if the request has been blocked due to safe browsing or parental control. This allows an administrator to easily filter based on lookups for NSFW websites or lookups that may indicate phishing or malware activity. If an administrator prefers to use filterlists to achieve or suppliment AGH's malware blocking or parental controls, these metrics will be incomplete or entirely unused.

AGH only categorizes lookups changed by it's own safe browsing or parental control features. While a user [may provide their own parental lookup or safe browsing server](https://github.com/AdguardTeam/AdGuardHome/blob/5e7e0092ac3b2f4bbdb30aebc5b67dc9e7995249/internal/home/config.go#L557) in the yaml config, the process of deploying such a system is not documented to my knowledge, nor would it be an ideal solution for a small deployment, such as on a home router or a raspberry pi.

### Proposed solution

A more appropriate solution would be for AGH to support categorizing filterlists as parental or security-related. This would match up with the current approach adopted by filter maintainers and *should* be fairly simple to implement

For custom rules, a `$agcategory` rule modifier could be added to tag one-off rules as parental or security-related.

### Alternatives considered and additional information

The unofficial `$reason` rule modifier is sometimes seen in filterlists, however it's used by maintainers to comment on rather than categorize a particular rule.