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.

common/account_utils/src/validator_definitions.rs

@@ -45,6 +45,29 @@ pub enum Error {
     UnableToCreateValidatorDir(PathBuf),
 }
 
+#[derive(Clone, PartialEq, Serialize, Deserialize, Hash, Eq)]
+pub struct Web3SignerDefinition {
+    pub url: String,
+    /// Path to a .pem file.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub root_certificate_path: Option<PathBuf>,
+    /// Specifies a request timeout.
+    ///
+    /// The timeout is applied from when the request starts connecting until the response body has finished.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub request_timeout_ms: Option<u64>,
+
+    /// Path to a PKCS12 file.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub client_identity_path: Option<PathBuf>,
+
+    /// Password for the PKCS12 file.
+    ///
+    /// An empty password will be used if this is omitted.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub client_identity_password: Option<String>,
+}
+
 /// Defines how the validator client should attempt to sign messages for this validator.
 #[derive(Clone, PartialEq, Serialize, Deserialize)]
 #[serde(tag = "type")]
@@ -62,27 +85,7 @@ pub enum SigningDefinition {
     ///
     /// https://github.com/ConsenSys/web3signer
     #[serde(rename = "web3signer")]
-    Web3Signer {
-        url: String,
-        /// Path to a .pem file.
-        #[serde(skip_serializing_if = "Option::is_none")]
-        root_certificate_path: Option<PathBuf>,
-        /// Specifies a request timeout.
-        ///
-        /// The timeout is applied from when the request starts connecting until the response body has finished.
-        #[serde(skip_serializing_if = "Option::is_none")]
-        request_timeout_ms: Option<u64>,
-
-        /// Path to a PKCS12 file.
-        #[serde(skip_serializing_if = "Option::is_none")]
-        client_identity_path: Option<PathBuf>,
-
-        /// Password for the PKCS12 file.
-        ///
-        /// An empty password will be used if this is omitted.
-        #[serde(skip_serializing_if = "Option::is_none")]
-        client_identity_password: Option<String>,
-    },
+    Web3Signer(Web3SignerDefinition),
 }
 
 impl SigningDefinition {