Add option to get reign for ytserver-all

[l0kix2]

It would be useful for ytsaurus/yt-k8s-operator#200 to compare old and new image of master reign.

Something like /usr/bin/ytserver-all --reign would do.

[koct9i]

Let's add one flat kv/json/yson structure which could contain all reigns, yt verison, rpc protocol versions, etc.

[zlobober]

Let's do it in simple form only for master reigns for now.
Only for ytserver-master, let's add a flag "--compatibility-info", which prints a single YSON of form:

{
    "current_reign" = 1234;
}

Relevant code:

• A function for getting current reign lives https://github.com/ytsaurus/ytsaurus/blob/50fca7689f09efc4e2ccdd16e8a8353f70a6ee90/yt/yt/server/master/cell_master/serialize.h#L33.
• A code handling command-line arguments for ytserver-master lives here
• An example of dumping some information in YSON lives here