Take abseil as a dependency

1. Add abseil-cpp as a submodule. We are tracking the latest LTS
release, which is lts_2024_01_16.
2. Replace glog/gflags with absl::log and absl::flags.
3. Remove miniglog
4. Also take a whack at making the bazel build work with
   abseil-cpp and gtest.

There are a number of TODOs in this CL that still need to be resolved.

Change-Id: I39355ed7d61375be4ebcbc8596d9cc70acc1c678

examples/helloworld_numeric_diff.cc

@@ -31,8 +31,8 @@
 // Minimize 0.5 (10 - x)^2 using jacobian matrix computed using
 // numeric differentiation.
 
+#include "absl/log/initialize.h"
 #include "ceres/ceres.h"
-#include "glog/logging.h"
 
 // A cost functor that implements the residual r = 10 - x.
 struct CostFunctor {
@@ -43,7 +43,7 @@ struct CostFunctor {
 };
 
 int main(int argc, char** argv) {
-  google::InitGoogleLogging(argv[0]);
+  absl::InitializeLog();
 
   // The variable to solve for with its initial value. It will be
   // mutated in place by the solver.