Cache header values at the start of request processing in chain listeners
instead of accessing them multiple times via fiber.(*Ctx).GetReqHeaders
and fiber.(*Ctx).Get().

• Add GetHeaderFromCachedMap helper for extracting headers from cached map
• Add ExtractWantedHeadersFromCachedMap to extract context headers from cached map
• Update jsonRPC, rest, and tendermintRPC handlers to cache headers early
• Fix redundant fiberCtx.Get() call in rest.go handlerUse

This reduces per-request allocations by consolidating multiple header
access calls into a single GetReqHeaders() call with map lookups.

Description

Closes: #XXXX

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