Replace serde_yaml with serde_json internally

[rezvaneh]

We replaced serde_yaml with serde_json to simplify writing tests and conversion to protobuf

Which issue(s) this PR fixes:
#507

[bombsimon]

This surely looks way more convenient, LGTM!

[XAMPPRocky]

LGTM! Just need to run cargo fmt and fix the cargo clippy lints, and it should be good to go.

[rezvaneh]

LGTM! Just need to run cargo fmt and fix the cargo clippy lints, and it should be good to go.

I had ran them before, can you see what error says?

[XAMPPRocky]

Step #4 - "test-quilkin": error: casting integer literal to `i64` is unnecessary
Step #4 - "test-quilkin":    --> src/filters/capture.rs:144:25
Step #4 - "test-quilkin":     |
Step #4 - "test-quilkin": 144 |                 "size": 3 as i64,
Step #4 - "test-quilkin":     |                         ^^^^^^^^ help: try: `3_i64`
Step #4 - "test-quilkin":     |
Step #4 - "test-quilkin":     = note: `-D clippy::unnecessary-cast` implied by `-D warnings`
Step #4 - "test-quilkin":     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
Step #4 - "test-quilkin": 
Step #4 - "test-quilkin": error: casting integer literal to `i64` is unnecessary
Step #4 - "test-quilkin":    --> src/filters/capture.rs:162:25
Step #4 - "test-quilkin":     |
Step #4 - "test-quilkin": 162 |                 "size": 3 as i64,
Step #4 - "test-quilkin":     |                         ^^^^^^^^ help: try: `3_i64`
Step #4 - "test-quilkin":     |
Step #4 - "test-quilkin":     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
Step #4 - "test-quilkin": 
Step #4 - "test-quilkin": error: useless conversion to the same type: `&str`
Step #4 - "test-quilkin":    --> src/xds/cluster.rs:785:38
Step #4 - "test-quilkin":     |
Step #4 - "test-quilkin": 785 |         let value = dyn_metadata.get("key-a".into()).unwrap();
Step #4 - "test-quilkin":     |                                      ^^^^^^^^^^^^^^ help: consider removing `.into()`: `"key-a"`
Step #4 - "test-quilkin":     |
Step #4 - "test-quilkin":     = note: `-D clippy::useless-conversion` implied by `-D warnings`
Step #4 - "test-quilkin":     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
Step #4 - "test-quilkin": 
Step #4 - "test-quilkin": error: could not compile `quilkin` due to 3 previous errors
Step #4 - "test-quilkin": make: *** [Makefile:55: test-quilkin] Error 101
Finished Step #4 - "test-quilkin"

[quilkin-bot]

Build Succeeded 🥳

Build Id: 3a8b1bcd-c679-464a-bb54-c17ed18004c4

To build this version:

git fetch [email protected]:googleforgames/quilkin.git pull/518/head:pr_518 && git checkout pr_518
cargo build