Add RM_GetContextUser api

[TalBarYakar]

Add RM_GetContextUser api to retrieve the RedisModuleUser set via RM_SetContextUser, allowing modules to access the user associated with RM_Call ACL checks.

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Note

Low Risk
Low-risk header-only API surface change; main concern is ABI/compatibility for modules running against older Redis versions where RedisModule_GetContextUser may be unavailable.

Overview
Adds a new module API entrypoint, RedisModule_GetContextUser, to retrieve the RedisModuleUser previously set on a context via RedisModule_SetContextUser.

Updates RedisModule_Init to resolve and expose this function via REDISMODULE_GET_API(GetContextUser) so modules can read the context user (e.g., for ACL-aware RM_Call flows).

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