Allow ConfigOption to be specified as const

This is the first part of two commits that attempts to simplify
the API that is used to retrieve configuration values.

Previously, we encouraged the following pattern to build a plugin.

```rust
let configured_plugin =
  Builder::new(
      tokio::io::stdin(),
      tokio::io::stdout())
  .option(ConfigOption::new_i64_with_default("some-option", 0, "Description"))
  .configure();

let value = configured_plugion.option("some-option");
match value {
  Some(Integer(i)) => {}, // Config provided
  None => {}, 		  // No config set
  _ => {}, 	          // This should never happened
}
```

This commit helps to move to the following pattern

```rust
const SOME_OPTION : ConfigOption<i64> = ConfigOption::new_i64_with_default(
                           "some-option",
			   "description");

async fn main() -> Result<()> {
    let plugin = Builder::new(tokio::io::stdin(), tokio::io::stdoout())
    	.option(SOME_OPTION)
	.configure()
	.await?;

    let value : i64 = plugin.option(SOME_OPTION)?;
}
```

plugins/examples/cln-plugin-startup.rs

@@ -12,15 +12,15 @@ async fn main() -> Result<(), anyhow::Error> {
     let state = ();
 
     if let Some(plugin) = Builder::new(tokio::io::stdin(), tokio::io::stdout())
-        .option(
-            options::ConfigOption::new_i64_with_default(
-                "test-option",
-                42,
-                "a test-option with default 42",
-            )
-            .build(),
-        )
-        .option(options::ConfigOption::new_opt_i64("opt-option", "An optional option").build())
+        .option(options::ConfigOption::new_i64_with_default(
+            "test-option",
+            42,
+            "a test-option with default 42",
+        ))
+        .option(options::ConfigOption::new_i64_no_default(
+            "opt-option",
+            "An optional option",
+        ))
         .rpcmethod("testmethod", "This is a test", testmethod)
         .rpcmethod(
             "testoptions",