Add MagicSigner to inject into polkadot api instance

[octave08]

📦 Pull Request

[Provide a general summary of the pull request here.]

• adding MagicSigner to inject into polkadot API instance so that a developer can integrate with Magic like the below easily

const magicSigner = new MagicSigner(magic)
const api = new ApiPromise(wsProvider, { signer: magicSigner });

api.tx.balances
  .transfer('5C5555yEXUcmEJ5kkcCMvdZjUo7NGJiQJMS7vZXEeoMhj3VQ', 123456)
  .signAndSend(SENDER, (status) => { ... });

✅ Fixed Issues

• [List any fixed issues here like: Fixes #XXXX]

🚨 Test instructions

[Describe any additional context required to test the PR/feature/bug fix.]

⚠️ Don't forget to add a semver label!

Please only add one label:

• patch: Bug Fix?
• minor: New Feature?
• major: Breaking Change?
• skip-release: It's unnecessary to publish this change.

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install @magic-ext/polkadot@16.3.0-canary.658.6732407355.0
# or 
yarn add @magic-ext/polkadot@16.3.0-canary.658.6732407355.0

[shortcut-integration]

This pull request has been linked to Shortcut Story #89834: [Polkadot] Add MagicSigner for Polkadot.