Enable serde feature on num-bigint

Summary: OSS buck2 bootstrap fails compiling buck2//pagable:pagable:

error[E0599]: the method serialize exists for reference &BigInt, but its trait bounds were not satisfied
  --> pagable/src/impls.rs:48:17

error[E0277]: the trait bound BigInt: serde::Deserialize<'de> is not satisfied
  --> pagable/src/impls.rs:57:12

pagable uses num-bigint::BigInt with serde traits via the serde feature of num-bigint. fbcode/buck2/Cargo.toml already has num-bigint = { version = “0.4.6”, features = [“serde”] }, but the OSS self-bootstrap shim Cargo.toml has num-bigint = “0.4.3” with no features, so the serde impls are not compiled into the shipped crate.

Add features = [“serde”] to the shim entry so it matches buck2’s cargo manifest.

Reviewed By: IanChilds

Differential Revision: D101813806

fbshipit-source-id: 7dec5a36ffe19a246d179355cc624e772442447d