Add subproject support for gpioplus

In the current state, the repository fails to build using subprojects
because gpioplus is marked as a hard dependency but is not defined as a
subproject. This mis

Add subproject support for gpioplus

In the current state, the repository fails to build using subprojects
because gpioplus is marked as a hard dependency but is not defined as a
subproject. This missing dependency breaks the compilation process. By
leveraging subproject support, we can include the necessary dependency
and ensure successful compilation.

Tested By:

Was able to successfully build state-manager on x86 linux using the
subprojects.

Change-Id: I80e29b384bbadf2d65723ea5d17f4dd4f5978b32
Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>

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