Role: add NeoFS alphabet node role

[roman-khimov]

This is needed to implement NeoFS governance scheme (#2267).

[erikzhang]

InnerRing?

[roman-khimov]

They're IR nodes, yes, but at the same time they're not all of IR (a subset of full IR). We do call this particular set of nodes "alphabet" nodes throughout the documentation already, so maybe it's OK to call them the same way here.

[erikzhang]

How do other IRs come about?

[roman-khimov]

As per governance scheme they're added/removed dynamically:

Inner Ring nodes follow a self regulation process, allowing them to vote for substitution of dead or malfunctioning nodes with new ones from the candidate list. Only Alphabet nodes prepare new InnerRing nodes lists and vote for it, but all nodes listed there must confirm their participation via the samevoting mechanism.