[cpp-rest-sdk-client] handling datetime type

[fedeitc]

A specialization for the shared_ptr<datetime> data type was added to
ModelBase member functions fromString, fromJson, toJson and
toHttpContent.

This should fix compilation errors on generated source code.
See https://github.com/coinapi/coinapi-sdk/pull/130

@muttleyxd

[wing328]

cc @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)

[wing328]

Tested locally with the updated samples and the result is good.

/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
Scanning dependencies of target CppRestPetstoreClient
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[  5%] Building CXX object CMakeFiles/CppRestPetstoreClient.dir/src/api/PetApi.cpp.o
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[ 11%] Linking CXX static library libCppRestPetstoreClient.a
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[100%] Built target CppRestPetstoreClient
/usr/bin/cmake: /usr/local/lib/libcurl.so.4: no version information available (required by /usr/bin/cmake)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  37.545 s
[INFO] Finished at: 2022-04-30T17:40:44+08:00
[INFO] ------------------------------------------------------------------------