Add verifymessage feature

In rust-bitcoin#326 we changed a function to use
`bitcoin::sign_message::MessageSignature` but doing so requires the
"base64" feature to be on for `bitcoin`. This did not get caught by CI
but improvements to CI in rust-bitcoin#338 will now catch this.

Add a feature to `json` and `client` that allows enabling "base64" if
the `verifymessage` RPC call is required.

Author: tcharding

Diff for: client/Cargo.toml

@@ -18,6 +18,9 @@ edition = "2018"
 name = "bitcoincore_rpc"
 path = "src/lib.rs"
 
+[features]
+verifymessage = ["bitcoincore-rpc-json/verifymessage"]
+
 [dependencies]
 bitcoincore-rpc-json = { version = "0.18.0", path = "../json" }
 
@@ -31,3 +34,10 @@ serde_json = "1"
 [dev-dependencies]
 tempfile = "3.3.0"
 
+[[example]]
+name = "retry_client"
+required-features = ["verifymessage"]
+
+[[example]]
+name = "test_against_node"
+required-features = []

Diff for: client/src/client.rs

@@ -871,6 +871,7 @@ pub trait RpcApi: Sized {
         self.call("stop", &[])
     }
 
+    #[cfg(feature = "verifymessage")]
     fn verify_message(
         &self,
         address: &Address,

Diff for: json/Cargo.toml

@@ -19,6 +19,9 @@ rust-version = "1.56.1"
 name = "bitcoincore_rpc_json"
 path = "src/lib.rs"
 
+[features]
+verifymessage = ["bitcoin/base64"]
+
 [dependencies]
 serde = { version = "1", features = [ "derive" ] }
 serde_json = "1"