Add timezone under custom time formats.

[pch3]

Is your feature request related to a problem? Please describe.
I use convert timestamp to custom format feature and it works great - thank you.
However to get the target format in UTC, I have to use timing.epochToCustomTimezone and always go via "additional" step of selecting the time zone.

Describe the solution you'd like
I proposed to add timezone key to custom formats, which would specify time zone which should be used. If no timezone is provided, "local" would be the default.

"timing.customFormats": [
        {
            "format": "YYYY-MM-DD HH:mm:ss",
            "detail": "My custom format #1",
            "timezone": "+00:00"
        },
        {
            "format": "YYYY-MM-DDTHH:mm:ss",
            "detail": "My custom format #2",
            "timezone": "+00:00"
        }
    ],

Describe alternatives you've considered
Alternativly, this new key could be used as a default value for 3rd step in timing.epochToCustomTimezone

Additional context
none