[SYCL][FPGA] Expose value_type and min_capacity from SYCL pipes extension class

[tyoungsc]

This is a convenience for users who may template their functions/classes on SYCL pipe types.
This allows them to do something like:

template<typename MyPipe>
void foo(/* ... */) {
    using PipeT = MyPipe::value_type;
    // ...
}

Test update: intel/llvm-test-suite#800

[againull]

I am sorry for the late review.