Share reqwest::Client between validators when using Web3Signer (#3335)

## Issue Addressed

#3302

## Proposed Changes

Move the `reqwest::Client` from being initialized per-validator, to being initialized per distinct Web3Signer. 
This is done by placing the `Client` into a `HashMap` keyed by the definition of the Web3Signer as specified by the `ValidatorDefintion`. This will allow multiple Web3Signers to be used with a single VC and also maintains backwards compatibility.

## Additional Info

This was done to reduce the memory used by the VC when connecting to a Web3Signer.

I set up a local testnet using [a custom script](https://github.com/macladson/lighthouse/tree/web3signer-local-test/scripts/local_testnet_web3signer) and ran a VC with 200 validator keys:


VC with Web3Signer:
- `unstable`: ~200MB
- With fix: ~50MB



VC with Local Signer:
- `unstable`: ~35MB
- With fix: ~35MB 


> I'm seeing some fragmentation with the VC using the Web3Signer, but not when using a local signer (this is most likely due to making lots of http requests and dealing with lots of JSON objects). I tested the above using `MALLOC_ARENA_MAX=1` to try to reduce the fragmentation. Without it, the values are around +50MB for both `unstable` and the fix.

testing/web3signer_tests/src/lib.rs

@@ -15,7 +15,7 @@
 #[cfg(all(test, unix, not(debug_assertions)))]
 mod tests {
     use account_utils::validator_definitions::{
-        SigningDefinition, ValidatorDefinition, ValidatorDefinitions,
+        SigningDefinition, ValidatorDefinition, ValidatorDefinitions, Web3SignerDefinition,
     };
     use eth2_keystore::KeystoreBuilder;
     use eth2_network_config::Eth2NetworkConfig;
@@ -376,13 +376,13 @@ mod tests {
                     graffiti: None,
                     suggested_fee_recipient: None,
                     description: String::default(),
-                    signing_definition: SigningDefinition::Web3Signer {
+                    signing_definition: SigningDefinition::Web3Signer(Web3SignerDefinition {
                         url: signer_rig.url.to_string(),
                         root_certificate_path: Some(root_certificate_path()),
                         request_timeout_ms: None,
                         client_identity_path: Some(client_identity_path()),
                         client_identity_password: Some(client_identity_password()),
-                    },
+                    }),
                 };
                 ValidatorStoreRig::new(vec![validator_definition], spec).await
             };