Reexport AddExtensionLayer type from tower_http in server generated crates

[paulnice]

Please reexport tower_http structures used in server generate crates as parts of the crates.

Recent updates in aws-smithy-http-server = 0.55 broke our code which directly imported aws-smithy-http-server = 0.54.4 and transitively import aws-smithy-http-server = 0.55 as part of server generated crate. The solution was to use the server generated imports of aws-smithy-http-server introduced in #1909.

But while doing a fix I found that AddExtensionLayer from tower_http is not reexported in the server generated crate and has a chance to broke our code again, because now I have to import tower_http directly in my Cargo.toml, instead of importing from aws-smithy-http-server.