Allow modern target_link_libraries with ament_cmake_gtest

[nwn]

As described in #405, the ament_add_gtest() and ament_add_gtest_executable() macros don't support subsequent calls to target_link_libraries() on the produced target with scope keywords (PRIVATE/PUBLIC/INTERFACE). Modern standards prefer to use scope keywords, so we should allow this.

CMake requires that either all or no calls to target_link_libraries() for a given target use scope keywords (CMP0023), so for backwards compatibility to avoid breaking downstream packages, the old behaviour is kept as a default for now, with the ability to opt in to the new behaviour by passing in LINKING_MODE SCOPED to ament_add_gtest() and friends. Use of the old method will now emit a deprecation warning, like:

CMake Deprecation Warning at /root/ws/install/ament_cmake_gtest/share/ament_cmake_gtest/cmake/ament_add_gtest_executable.cmake:64 (message):
  The unscoped signature for ament_add_gtest_executable() is deprecated.
  Instead, pass 'LINKING_MODE SCOPED' to ament_add_gtest_executable() and
  update any related target_link_libraries() calls to use scope keywords
  (PRIVATE/PUBLIC/INTERFACE).  This will eventually become the default.
Call Stack (most recent call first):
  /root/ws/install/ament_cmake_gtest/share/ament_cmake_gtest/cmake/ament_add_gtest_executable.cmake:40 (_ament_add_gtest_executable)
  /root/ws/install/ament_cmake_gtest/share/ament_cmake_gtest/cmake/ament_add_gtest.cmake:74 (ament_add_gtest_executable)
  CMakeLists.txt:146 (ament_add_gtest)

Eventually, after giving downstream users time to migrate, we can switch the default behaviour to SCOPED.

See also #423.

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[skyegalaxy]

@cottsay or @asymingt - can you take a look?

[Zyrin]

If you want to make the scoped mode the default, eventually (as stated in the warning), why not add something like SCOPED_LINKING_MODE as an option, instead of the one_value_keyword LINKING_MODE (with its keywords SCOPED and UNSCOPED)?
For the UNSCOPED mode ament_add_gtest would remain as is (except for a warning that the behavior will change in the future).
Only if explicit SCOPED mode is required one would add SCOPED_LINKING_MODE (or to silence the warning).
After the deprecation period is over only SCOPED_LINKING_MODE would need to be removed.

[nwn]

[...] why not add something like SCOPED_LINKING_MODE as an option, instead of the one_value_keyword LINKING_MODE (with its keywords SCOPED and UNSCOPED)?

@Zyrin I think the two approaches (single-value keyword or option) would be equivalent; either the default remains as LINKING_MODE UNSCOPED and you would use LINKING_MODE SCOPED to override it to the new way, or the default remains as UNSCOPED_LINKING_MODE and you use SCOPED_LINKING_MODE to override it.

I used a single-value keyword to indicate that only one of the two modes can be specified, but we could also just emit an error if the user specifies both the UNSCOPED_LINKING_MODE and SCOPED_LINKING_MODE options in the call. I'm fine with either approach.