Make find_package work for rmqcpp

[ahsanabbas123]

Problem statement

Make find_package(rmqcpp REQUIRED) work for applications installing rmqcpp using a package manager like vcpkg.

Proposed changes

• Remove absolute paths, export rmqcppTargets.cmake and rmqcppConfig.cmake [generated using the Config.cmake.in] files.
• Tested in an example application that builds rmqperftest with a local vcpkg registry overlay with a rmqcpp port to install rmqcpp.
• Reference cmake.in file used by bde-tools: https://github.com/bloomberg/bde-tools/blob/main/BdeBuildSystem/support/uorConfig.cmake.in
• Relevant cmake docs - https://cmake.org/cmake/help/latest/guide/importing-exporting/index.html