json: Enable base64 feature

We need base64 encoding in `bitcoin` to in order to stringify the
`sign_message::MessageSignature`.

Note also that the `Psbt` type is encoded/decoded in base64 and we would
like to support that by default also.

Enable the "base64" feature unconditionally for the `json` crate.

Author: tcharding

json/Cargo.toml

@@ -23,4 +23,4 @@ path = "src/lib.rs"
 serde = { version = "1", features = [ "derive" ] }
 serde_json = "1"
 
-bitcoin = { version = "0.31.0", features = ["serde", "rand-std"]}
+bitcoin = { version = "0.31.0", features = ["serde", "rand-std", "base64"]}