Support list configuration values via JSON arrays (5.x backport of #27)

[snake14]

Description

Backport of #27 and #29 to 5.x: environment variables holding a JSON array are now used as list values, so settings such as trusted_hosts can be set from the environment. JSON objects are decoded into keyed arrays; any other value is used literally, as before.

The decode uses plain json_decode() rather than JSON_THROW_ON_ERROR/JsonException, which 5.x's PHP 7.2 floor doesn't have; the behaviour is the same.

Issue No

Fixes #9 on 5.x (backport of #27 and #29)

Steps to Replicate the Issue

1. Set e.g. MATOMO_GENERAL_TRUSTED_HOSTS=["192.168.1.1","192.168.2.1"]
2. Expected: the configured trusted hosts are respected
3. Actual: they are ignored, since the value is a string rather than an array

Checklist

• [✔] Tested locally or on demo2/demo3?
• [NA] New test case added/updated?
• [NA] Are all newly added texts included via translation?
• [NA] Are text sanitized properly? (Eg use of v-text v/s v-html for vue)
• [✔] Version bumped?
• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules
• [✔] Documentation updated?

[inflatador]

@snake14 @mbrodala On behalf of the Wikimedia Foundation, thank you very much for the quick turnaround!