Support duplicate keys in HTTP API query strings (#2908)

## Issues Addressed

Closes #2739
Closes #2812

## Proposed Changes

Support the deserialization of query strings containing duplicate keys into their corresponding types.
As `warp` does not support this feature natively (as discussed in #2739), it relies on the external library [`serde_array_query`](https://github.com/sigp/serde_array_query) (written by @michaelsproul)

This is backwards compatible meaning that both of the following requests will produce the same output:
```
curl "http://localhost:5052/eth/v1/events?topics=head,block"
```
```
curl "http://localhost:5052/eth/v1/events?topics=head&topics=block"
```

## Additional Info

Certain error messages have changed slightly.  This only affects endpoints which accept multiple values.
For example:
```
{"code":400,"message":"BAD_REQUEST: invalid query: Invalid query string","stacktraces":[]}
```
is now
```
{"code":400,"message":"BAD_REQUEST: unable to parse query","stacktraces":[]}
```


The serve order of the endpoints `get_beacon_state_validators` and `get_beacon_state_validators_id` have flipped:
```rust
.or(get_beacon_state_validators_id.boxed())
.or(get_beacon_state_validators.boxed())
``` 
This is to ensure proper error messages when filter fallback occurs due to the use of the `and_then` filter.

## Future Work
- Cleanup / remove filter fallback behaviour by substituting `and_then` with `then` where appropriate.
- Add regression tests for HTTP API error messages.

## Credits
- @mooori for doing the ground work of investigating possible solutions within the existing Rust ecosystem.
- @michaelsproul for writing [`serde_array_query`](https://github.com/sigp/serde_array_query) and for helping debug the behaviour of the `warp` filter fallback leading to incorrect error messages.

common/warp_utils/Cargo.toml

@@ -18,3 +18,4 @@ tokio = { version = "1.14.0", features = ["sync"] }
 headers = "0.3.2"
 lighthouse_metrics = { path = "../lighthouse_metrics" }
 lazy_static = "1.4.0"
+serde_array_query = "0.1.0"

common/warp_utils/src/lib.rs

@@ -3,5 +3,6 @@
 
 pub mod cors;
 pub mod metrics;
+pub mod query;
 pub mod reject;
 pub mod task;

common/warp_utils/src/query.rs

@@ -0,0 +1,22 @@
+use crate::reject::custom_bad_request;
+use serde::Deserialize;
+use warp::Filter;
+
+// Custom query filter using `serde_array_query`.
+// This allows duplicate keys inside query strings.
+pub fn multi_key_query<'de, T: Deserialize<'de>>(
+) -> impl warp::Filter<Extract = (Result<T, warp::Rejection>,), Error = std::convert::Infallible> + Copy
+{
+    raw_query().then(|query_str: String| async move {
+        serde_array_query::from_str(&query_str).map_err(|e| custom_bad_request(e.to_string()))
+    })
+}
+
+// This ensures that empty query strings are still accepted.
+// This is because warp::filters::query::raw() does not allow empty query strings
+// but warp::query::<T>() does.
+fn raw_query() -> impl Filter<Extract = (String,), Error = std::convert::Infallible> + Copy {
+    warp::filters::query::raw()
+        .or(warp::any().map(String::default))
+        .unify()
+}