Make `ClientBuilder` directly accept boxed handler types #6375

Job	Run time
Build docs	26s
Check (unstable + typesize)	53s
Check (unstable (no default features))	31s
Check minimal versions	31s
Check (temp cache)	44s
Check (builder without model)	28s
Check (temp cache + typesize)	56s
Examples	2m 19s
Test (native-tls)	57s
Test (no default features)	42s
Test (stable)	1m 3s
Test (nightly)	42s
Test (macOS)	43s
Test (no gateway)	53s
Test (no cache)	51s
Test (beta)	38s
Test (zlib compression)	1m 4s
Test (zstd compression)	58s
Test (Windows)	2m 10s
Test (chrono)	41s
Test (zlib and zstd compression)	1m 0s
MSRV	-6s
	19m 4s

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