node: Optimize verification of the request signatures

Is your feature request related to a problem? Please describe.

NeoFS API server sometimes wastes large amounts of memory. For example, to verify request signatures, the server decodes messages into intermediate structures, encodes its parts (field-ordered Protocol Buffers V3), verifies signatures and request continues processing.

Describe the solution you'd like

do not perform extra decode-encode stages. To do this, we must arrange the fields in the message without an additional buffer, calculate the checksum and verify the signature. Nothing blocks us from this cuz Protobuf is pretty simple and gRPC lib provides a lot of utilities around it

Describe alternatives you've considered

leave as it is: simpler code but worse performance

Start by locating the Go code that verifies NeoFS request signatures and tracing its current decode, re-encode, checksum, and verification path. Read the gRPC utilities and Protocol Buffers handling used there. Done means the extra decode-encode stages and intermediate buffer are avoided while signature verification and request processing remain correct.