[FEA] Optionally compile rmm as object library #2463

Is your feature request related to a problem? Please describe.
When someone is creating a static library (e.g., libnvcomp_static.a), they might want to embed librmm.a into their own artifact. This is to simplify dependency management for users, especially, if rmm is only used for a subset of the library's functionality. This is basically the case for nvcomp, where we only use librmm in a subset of the functions and we don't want to force users to always have librmm available on their system.
During a static library creation, the linker is not invoked, so we cannot pull of the -Wl,--whole-archive trick that one can do when creating a shared library.

Describe the solution you'd like
Ideally we would like to have a cmake option to allow us to compile librmm into an OBJECT library.
On your side, the change is something along the lines of:

option(BUILD_OBJECT_LIBS "Configure CMake to build rmm as object library" OFF)

if(BUILD_OBJECT_LIBS)
    set(RMM_LIB_TYPE OBJECT)
elseif(BUILD_SHARED_LIBS)
    set(RMM_LIB_TYPE SHARED)
else()
    set(RMM_LIB_TYPE STATIC)
endif()

add_library(rmm ${RMM_LIB_TYPE} ...)

We could certainly run ar x librmm.a to get back the object files and embed those, but a lot more elegant way would be to embed rmm via $<TARGET_OBJECTS:rmm::rmm> once we compiled it as an object library.

Describe alternatives you've considered

• ar x librmm.a and then embedding the resulting object files