Update CMakeLists.txt for more portability (#1005)

add alias hiredis::hiredis and hiredis::hiredis_static so when this project is FetchContent, it behave the same as find_package-ed.
e.g.
```
find_package(hiredis 1.0.2 QUIET)
if (NOT hiredis_FOUND)
    FetchContent_Declare(
            hiredis
            GIT_REPOSITORY https://github.com/redis/hiredis.git
            GIT_TAG v1.0.2
    )
    FetchContent_MakeAvailable(hiredis)
endif ()
# later
target_link_libraries(target **hiredis::hiredis**) #can always use hiredis::hiredis to ref to this project. no matter find_package or FetchContent
```
this is actually sort of best practice when making a CMake lib.

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