hrw: forward declare template specializations

[cmcfarlen]

Older (but not that old) compiler's can't see template specializations implemented in another translation unit without forward declaring them. This adds the forward declarations and also adds a unit test that was used to track down and verify this issue.

This is an alternative fix to #12433

[maskit]

Looks good to me. I'll close #12433.

[bneradt]

It would be nice to land #12453 so we don't conflict with these.

[bneradt]

[approve ci]

[bneradt]

Here's the ubuntu build failure:
https://ci.trafficserver.apache.org/job/Github_Builds/job/ubuntu/6404/console

: && /usr/lib/llvm-12/bin/clang++ -D_FORTIFY_SOURCE=2 -fPIE -fstack-protector -pthread -g -pie -Wl,-z,relro -Wl,-z,now plugins/header_rewrite/CMakeFiles/test_matcher.dir/matcher_tests.cc.o plugins/header_rewrite/CMakeFiles/test_matcher.dir/matcher.cc.o plugins/header_rewrite/CMakeFiles/test_matcher.dir/lulu.cc.o plugins/header_rewrite/CMakeFiles/test_matcher.dir/regex_helper.cc.o plugins/header_rewrite/CMakeFiles/test_matcher.dir/resources.cc.o -o plugins/header_rewrite/test_matcher  -Wl,-rpath,/home/jenkins/workspace/Github_Builds/ubuntu/src/build/lib/swoc:/home/jenkins/workspace/Github_Builds/ubuntu/src/build/lib/yamlcpp  -ldl  src/tsutil/libtsutil.a  lib/swoc/libswoc.so.1.5.14  lib/yamlcpp/libyaml-cppd.so.0.8.0  -ldl  /usr/lib/x86_64-linux-gnu/libpcre2-8.so  /usr/lib/x86_64-linux-gnu/libpcre.so && :
/usr/bin/ld: plugins/header_rewrite/CMakeFiles/test_matcher.dir/matcher_tests.cc.o: in function `ProxyMutex::free()':
/home/jenkins/workspace/Github_Builds/ubuntu/src/build/../include/iocore/eventsystem/Lock.h:657: undefined reference to `ink_mutex_destroy(pthread_mutex_t*)'
/usr/bin/ld: /home/jenkins/workspace/Github_Builds/ubuntu/src/build/../include/iocore/eventsystem/Lock.h:658: undefined reference to `mutexAllocator'
/usr/bin/ld: plugins/header_rewrite/CMakeFiles/test_matcher.dir/matcher_tests.cc.o: in function `FreelistAllocator::free_void(void*)':
/home/jenkins/workspace/Github_Builds/ubuntu/src/build/../include/tscore/Allocator.h:73: undefined reference to `ink_freelist_free(_InkFreeList*, void*)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)