Build dependencies as external projects

[ashao]

Previously we were using a Makefile to download and install third-party dependencies for the library compilation (hiredis, redis++, and PyBind). This PR migrates this functionality to the CMakeLists.txt by adding hiredis and redis++ as ExternalProjects and by using cmake to download the pybind headers.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.24%. Comparing base (8613627) to head (72b4d71).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #497       +/-   ##
============================================
- Coverage    95.92%   78.24%   -17.69%     
============================================
  Files           10       90       +80     
  Lines          884     6867     +5983     
============================================
+ Hits           848     5373     +4525     
- Misses          36     1494     +1458     

see 80 files with indirect coverage changes

[mellis13]

Thank you for making SmartSim compatible as an external project!