client: Add dedicated parameters for well-known X-Headers

[cthulhu-rider]

• based on and blocked by Clarification for eXtended Headers in MetaHeader neofs-api#204
• partially touches Make working with key-value pairs more convenient #167

Overview

Some well-known X-Headers can affect RPC execution. It'd convenient way to parameterize these values using dedicated methods.

Proposal

Implement methods of PrmObjectGet, PrmObjectRange, PrmObjectSearch and PrmObjectHead types

func (x *PrmX) ProcessEpoch(uint64)

func (x *PrmX) LookBackToEpoch(uint64)