feat(rpc): expose reqwest client

Previously, the reqwest client for HttpClient would be unconditionally built
by the internal builder, meaning that useful middleware such as tower could
not be applied.

This commits adds two ways to create the HttpClient with a custom reqwest::Client,
either through new_from_parts, or using the builder.

rpc/src/client/transport/http.rs

@@ -62,6 +62,7 @@ pub struct Builder {
     compat: CompatMode,
     proxy_url: Option<HttpClientUrl>,
     timeout: Duration,
+    client: Option<reqwest::Client>,
 }
 
 impl Builder {
@@ -93,22 +94,35 @@ impl Builder {
         self
     }
 
+    /// Use the provided client instead of building one internally.
+    pub fn client(mut self, client: reqwest::Client) -> Self {
+        self.client = Some(client);
+        self
+    }
+
     /// Try to create a client with the options specified for this builder.
     pub fn build(self) -> Result<HttpClient, Error> {
-        let builder = reqwest::ClientBuilder::new()
-            .user_agent(USER_AGENT)
-            .timeout(self.timeout);
-        let inner = match self.proxy_url {
-            None => builder.build().map_err(Error::http)?,
-            Some(proxy_url) => {
-                let proxy = if self.url.0.is_secure() {
-                    Proxy::https(reqwest::Url::from(proxy_url.0)).map_err(Error::invalid_proxy)?
-                } else {
-                    Proxy::http(reqwest::Url::from(proxy_url.0)).map_err(Error::invalid_proxy)?
-                };
-                builder.proxy(proxy).build().map_err(Error::http)?
-            },
+        let inner = if let Some(inner) = self.client {
+            inner
+        } else {
+            let builder = reqwest::ClientBuilder::new()
+                .user_agent(USER_AGENT)
+                .timeout(self.timeout);
+            match self.proxy_url {
+                None => builder.build().map_err(Error::http)?,
+                Some(proxy_url) => {
+                    let proxy = if self.url.0.is_secure() {
+                        Proxy::https(reqwest::Url::from(proxy_url.0))
+                            .map_err(Error::invalid_proxy)?
+                    } else {
+                        Proxy::http(reqwest::Url::from(proxy_url.0))
+                            .map_err(Error::invalid_proxy)?
+                    };
+                    builder.proxy(proxy).build().map_err(Error::http)?
+                },
+            }
         };
+
         Ok(HttpClient {
             inner,
             url: self.url.into(),
@@ -118,6 +132,13 @@ impl Builder {
 }
 
 impl HttpClient {
+    /// Construct a new Tendermint RPC HTTP/S client connecting to the given
+    /// URL. This avoids using the `Builder` and thus does not perform any
+    /// validation of the configuration.
+    pub fn new_from_parts(inner: reqwest::Client, url: reqwest::Url, compat: CompatMode) -> Self {
+        Self { inner, url, compat }
+    }
+
     /// Construct a new Tendermint RPC HTTP/S client connecting to the given
     /// URL.
     pub fn new<U>(url: U) -> Result<Self, Error>