feat: export checkout_session_ext #1119

Job	Run time
clippy-codegen	30s
format	26s
generate-public-api	1m 34s
clippy (async-std-surf)	1m 33s
verify-codegen	57s
clippy (tokio-hyper)	1m 11s
test (async-std-surf)	4m 53s
clippy (tokio-hyper-rustls)	1m 5s
test (tokio-hyper)	3m 50s
clippy (tokio-hyper-rustls-webpki)	1m 16s
test (tokio-hyper-rustls)	4m 37s
test (tokio-hyper-rustls-webpki)	4m 47s
clippy (blocking)	1m 20s
test (blocking)	3m 44s
clippy (blocking-rustls)	1m 15s
test (blocking-rustls)	3m 48s
clippy (blocking-rustls-webpki)	1m 8s
test (blocking-rustls-webpki)	5m 2s
	42m 56s

Provide feedback