Add plain HTTP egress forwarding

[Weidong Cui (wdcui)]

This PR adds plain HTTP/1 forwarding to the standalone proxy, including absolute-to-origin request rewriting, canonical Host forwarding, hop-by-hop and framing validation, per-request upstream connections, bounded body streaming, and hermetic loopback coverage.

[github-actions]

ℹ️ Note: This semver check was run against the wdcui/ulitebox/egress-proxy-connect branch, not main or ulitebox.

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
  variant FramingError::DuplicateHost 0 -> 6 in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:52
  variant FramingError::BodyOnConnect 1 -> 7 in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:55

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/enum_variant_added.ron

Failed in:
  variant FramingError:ConflictingLength in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:34
  variant FramingError:ConflictingContentLength in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:37
  variant FramingError:InvalidContentLength in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:40
  variant FramingError:UnsupportedTransferCoding in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:43
  variant FramingError:TransferEncodingOnHttp10 in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:46
  variant FramingError:NonAsciiHeaderValue in /home/runner/work/litebox/litebox/litebox_egress_proxy/src/headers.rs:49