unify IP setting for RTPS, and separate platform dependent code from workspace

[takasehideki]

Previously, IP address settings were required in both ./include/config/rtps.h and in app.cpp, but I could solve this situation so that it can work with only one setting, in ./platform/mros2-platform.h.
After the network configuration is complete, the program obtains the IP address and stores it to the configuration for embeddedRTPS (rtps::Config::IP_ADDRESS). This may increase the implementation responsibility for each platform, but I believe it should be more convenient for the user.

We have confirmed that this works only with mros2-mbed / echoback_string. Please check along with the following.
mROS-base/mros2#45

Below is the list to be tested

• mandatory
  • echoback_string
  • echoreply_string
  • pub_float32
  • pub_twist
  • sub_pose
  • sub_uint16
  • pub_image
  • pub_long_string_sub_crc
• optional (platform dependent and/or needs additional equipment)
  • mturtle_teleop
  • mturtle_teleop_joy
  • pub_camera_image

[takasehideki]

@s-hosoai I added the list to be tested. I think this PR can be merged when all operations of "mandatory" can be confirmed.

[takasehideki]

I've checked all operations of "mandatory" examples! I've also confirmed the build of "optional" examples, but not confirmed the operations yet due to a lack of modules,,, Anyway, going to be merged!!