@polkadot/metadata conflicts

Simply using `@polkadot/api` as a dependency in a project with `@cennznet/api` seems to cause type decoding issues at runtime.

By setting a common version for `@polkadot/metadata` it fixes the issues
```js
{
//..
"dependencies": {
"@cennznet/api": "^1.3.2",
"@polkadot/api": "^2.7.1"
},
"resolutions": {
"@polkadot/metadata": "2.7.1"
}
// ..
}
```

simple initialization fails:
```js
// test.js
const { Api, WsProvider } = require('@cennznet/api');

let provider = new WsProvider('ws://example.com:9944');
```

---

Not sure this is a true bug but leaving here as reference