Restore enhanced stacktraces when libbacktrace is not found

[marcomagdy]

This PR restores the enhanced stack traces (with line numbers and source filenames) even when libbacktrace is not found.
It also ensures that Alpine would continue to work as expected since the package is now not "required".

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bmoffatt]

Trying to test this out - I added a CI workflow with an Alpine 3.19 container - which shows as expected that the crash test no longer passes on newer Alpine since backtrace isn't present. However these test cases fail to build if I merge this branch.

agent_1  | ninja: job failed: /usr/bin/c++ -DAWS_LAMBDA_LOG=3 -Daws_lambda_runtime_EXPORTS -I/codebuild/output/src226519862/src/include -g -std=gnu++11 -fPIC -fno-exceptions -fno-rtti -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -Werror -Wconversion -Wno-si
gn-conversion -MD -MT CMakeFiles/aws-lambda-runtime.dir/src/backward.cpp.o -MF CMakeFiles/aws-lambda-runtime.dir/src/backward.cpp.o.d -o CMakeFiles/aws-lambda-runtime.dir/src/backward.cpp.o -c /codebuild/output/src226519862/src/src/backward.cpp
agent_1  | In file included from /codebuild/output/src226519862/src/src/backward.cpp:26:
agent_1  | /codebuild/output/src226519862/src/src/backward.h:262:18: fatal error: execinfo.h: No such file or directory
agent_1  |   262 | #        include <execinfo.h>
agent_1  |       |                  ^~~~~~~~~~~~

Also tried adding apk add elfutils-dev per #124 (comment), but ran into a different error.

agent_1  | ninja: job failed: /usr/bin/c++ -DAWS_LAMBDA_LOG=3 -DBACKWARD_HAS_DW=1 -Daws_lambda_runtime_EXPORTS -I/codebuild/output/src601444743/src/include -g -std=gnu++11 -fPIC -fno-exceptions -fno-rtti -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -Werror
-Wconversion -Wno-sign-conversion -MD -MT CMakeFiles/aws-lambda-runtime.dir/src/backward.cpp.o -MF CMakeFiles/aws-lambda-runtime.dir/src/backward.cpp.o.d -o CMakeFiles/aws-lambda-runtime.dir/src/backward.cpp.o -c /codebuild/output/src601444743/src/src/backward.cpp
agent_1  | In file included from /codebuild/output/src601444743/src/src/backward.cpp:26:
agent_1  | /codebuild/output/src601444743/src/src/backward.h:1291:35: error: 'Dl_info' has not been declared
agent_1  |  1291 |     std::string resolve_exec_path(Dl_info& symbol_info) const
agent_1  |       |                                   ^~~~~~~
agent_1  | /codebuild/output/src601444743/src/src/backward.h: In member function 'std::string backward::TraceResolverLinuxBase::resolve_exec_path(int&) const':
agent_1  | /codebuild/output/src601444743/src/src/backward.h:1295:25: error: request for member 'dli_fname' in 'symbol_info', which is of non-class type 'int'
agent_1  |  1295 |         if (symbol_info.dli_fname == argv0_) {
agent_1  |       |                         ^~~~~~~~~
agent_1  | /codebuild/output/src601444743/src/src/backward.h:1308:25: error: request for member 'dli_fname' in 'symbol_info', which is of non-class type 'int'
agent_1  |  1308 |             symbol_info.dli_fname = "/proc/self/exe";
agent_1  |       |                         ^~~~~~~~~
agent_1  | /codebuild/output/src601444743/src/src/backward.h:1312:32: error: request for member 'dli_fname' in 'symbol_info', which is of non-class type 'int'
agent_1  |  1312 |             return symbol_info.dli_fname;
agent_1  |       |