mediasoup-node: Make appData TS typed and writable

[ibc]

• This PR replaces PR Add generic types for appData #869 by @mango-martin (credits to him).
• Fixes Make appData no longer read-only #1043

Usage (everywhere when creating a mediasoup entity):

const router = await worker.createRouter<{ lalala: number }(options);

const producer = await transport.produce<{ foo: number; bar?: string }(options);

[jmillan]

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