Consider using a template for GAPIC's config.cmake.in

[dbolduc]

A follow up to #12428 (our cmake yak has more hair)

These config.cmake.in files are basically all the same. The only difference is the name of the library, and some of them have extra dependencies. (e.g. asset must find_dependency(osconfig) and others).

#12482 has an example showing what this work would probably look like:

google-cloud-cpp/cmake/GoogleCloudCppLibrary.cmake

Lines 96 to 98 in d5382a0

	set(GOOGLE_CLOUD_CPP_CONFIG_LIBRARY "${library}")
	configure_file("${PROJECT_SOURCE_DIR}/cmake/templates/config.cmake.in"
	"google_cloud_cpp_${library}-config.cmake" @ONLY)

https://github.com/googleapis/google-cloud-cpp/blob/d5382a01e480c6dd58b31e670472381425c5dcb4/cmake/templates/config.cmake.in

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The gain is one less file per library created by the scaffolding and manually maintained by hand. (not that these change often)

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Categories of Libraries that do not perfectly match the template

Cross library dependencies (in GA libraries):

• asset -> accesscontextmanager, osconfig
• binaryauthorization, containeranalysis -> grafeas
• contentwarehouse -> documentai

Non-standard *-targets.cmake file name.

• pubsub
• spanner

REST-based (and weird, anyways):

• storage