Document resolvers on extra arguments

[lorisleiva]

This PR documents how to resolve the default of an extra argument with custom code. A codama.resolver plugin on a codama.extraArgument plugin node describes how renderers resolve that extra argument's default, as it would on a struct field.

pluginNode('codama.extraArgument', {
    payload: { identifier: 'tokenStandard', type: definedTypeLinkNode('tokenStandard') },
    plugins: [pluginNode('codama.resolver', { payload: { name: 'resolveTokenStandard', dependsOn: ['accounts.mint'] } })],
});

Upgrading a v1 IDL produces this shape when an extra argument's default is a resolver, or otherwise can't be expressed in v2, so that no resolver is lost in the upgrade.