Add `-of_objects` key to `get_lib_pins` arguments

[akashlevy]

This PR adds the -of_objects key argument to get_lib_pins for improved SDC processing.

I mimicked the strategy that was used to add -of_objects to get_lib_cells.

• [x] doc/OpenSTA.odt, doc/OpenSTA.pdf: Update docs
• [x] doc/messages.txt: Update messages (added 0128, 0129, 0335)
• [x] sdc/Sdc.tcl: Add key
• [x] tcl/CmdArgs.tcl: Add helper function get_libcells_error
• [x] test/get_lib_pins_of_objects.tcl, test/get_lib_pins_of_objects.ok, test/regression_vars.tcl
  • [x] Add test case for get_lib_pins -of_objects on a single lib pin
  • [x] Add test case for get_lib_pins -of_objects on multiple lib pins