Rust

Add support for foreign function callbacks. #2353

Job	Run time
bazel	1m 10s
licenses	14s
audit	21s
examples (hello_world)	21s
examples (http_auth_random)	24s
msrv	1m 46s
nightly	27s
examples (http_body)	16s
outdated	15s
examples (http_config)	24s
reactors (hello_world)	22s
examples (http_headers)	25s
stable	33s
examples (grpc_auth_random)	13s
reactors (http_auth_random)	24s
examples (envoy_filter_metadata)	17s
reactors (http_body)	20s
reactors (http_config)	21s
reactors (http_headers)	21s
reactors (grpc_auth_random)	19s
reactors (envoy_filter_metadata)	20s
	9m 33s

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