rust: add serde and serde_json dependencies (#4539)

Summary:
The [`serde`] crate is a key ecosystem framework, providing a bridge
between data formats (JSON, YAML, CBOR, etc.) and serializable structs.
A Rust struct can `#[derive(Serialize, Deserialize)]` and then operate
with any data format, such as [`serde_json`]. Most pressingly, we want
these for encoding structs within blob keys.

[`serde`]: https://crates.io/crates/serde
[`serde_json`]: https://crates.io/crates/serde_json

Test Plan:
They build: `bazel build //third_party/rust:{serde,serde_json}`.

wchargin-branch: rust-dep-serde

Author: wchargin

tensorboard/data/server/Cargo.lock

@@ -35,6 +35,8 @@ log = "0.4.11"
 prost = "0.6.1"
 rand = "0.7.3"
 rand_chacha = "0.2.2"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0.59"
 thiserror = "1.0.21"
 tokio = { version = "0.2.2", features = ["macros"] }
 tonic = "0.3.1"