[FEAT] Support Transmission 5 format

[asardaes]

Is this a new feature request?

• I have searched the existing issues

Wanted change

The code assumes rpc-port will be present in the settings JSON:

docker-transmission/root/etc/s6-overlay/s6-rc.d/svc-transmission/finish

Line 5 in 88093db

PORT=$(jq '.["rpc-port"]' /config/settings.json)

But that will change in v5, and users can already opt in by setting TR_SAVE_VERSION_FORMAT=5

Reason for change

As seen in transmission/transmission#7108, they want to align on snake case.

Proposed code change

I think using this would support both versions:

PORT=$(jq '.["rpc-port"] // .["rpc_port"]' /config/settings.json)

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