Make `ClientBuilder` directly accept boxed handler types #6376

Job	Run time
Check (unstable + typesize)	39s
Check (temp cache)	49s
Check minimal versions	34s
Check (builder without model)	30s
Check (unstable (no default features))	25s
Build docs	28s
Check (temp cache + typesize)	37s
Test (stable)	1m 1s
Test (nightly)	41s
Examples	2m 9s
Test (Windows)	1m 55s
Test (no cache)	1m 3s
Test (no gateway)	58s
Test (native-tls)	1m 1s
Test (no default features)	35s
Test (macOS)	40s
Test (zlib and zstd compression)	1m 50s
Test (chrono)	35s
Test (zstd compression)	59s
Test (beta)	43s
Test (zlib compression)	1m 4s
MSRV	0s
	19m 16s

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