Add cache-busting and server headers with a wrapper.

In order to support custom headers for various response types,
  this commit adds a wrapper type, ResponseWrapper, which can service
  all types of response in `bin`.

For paste objects, the preferred `Last-Modified` is used, so that caches
  can compare their exact timings with the HEAD response when
  revalidating.

For static objects, an `ETag` is used instead, based on the Cargo version
  and git hash of the codebase at compilation time; a `build.rs` is used
  for this.

Cargo.toml

@@ -3,6 +3,7 @@ name = "bin"
 version = "2.0.0"
 authors = ["Gunwant Jain <mail@wantguns.dev>"]
 edition = "2021"
+build = "build.rs"
 
 [dependencies]
 rand = "0.8.4"
@@ -11,6 +12,9 @@ tree_magic = "0.2.3"
 syntect = "4.6.0"
 rust-embed="6.3.0"
 clap = { version = "3.0.9", features = ["derive"] }
+once_cell = "1"
+sha256 = "1"
+time = { version = "0.3", features = ["formatting"] }
 
 [dependencies.rocket_dyn_templates]
 version = "0.1.0-rc.1"