Resolve installed Boost dependencies from the producer configuration

[dajiaohuang]

Problem

Installing with CLI_UseBoostAsio=ON and compiled Boost libraries succeeds, but a fresh consumer using only find_package(cli CONFIG REQUIRED) and cli::cli fails CMake generation: the exported link interface references Boost::system without defining that imported target. It also exports Boost::date_time.

The package config tests the consumer's CLI_UseBoostAsio, rather than the configuration that created the exported target. This revisits the installed-package behavior discussed in #117, including the current header-only fallback.

Change

Record whether compiled Boost targets were exported, substitute that fact into the installed config, and discover both exported Boost components. Do not require compiled Boost libraries for the header-only fallback or no-Asio configuration.

Add an install-and-consume CTest covering compiled Boost, header-only Boost, and no Boost modes.

Validation

• Fresh installed consumer fails on master 769c5fa with missing Boost::system; succeeds after the change.
• All three install/consumer configurations passed on Ubuntu 24.04 / CMake 3.28 / GCC 13 / Boost 1.83.
• Six Boost-enabled examples compiled; a public installed consumer executed its addition command successfully.
• Windows/macOS, old CMake, standalone-Asio and the full CI matrix were not run locally.