KeyPair: add fromBytes, toEd25519PrivateKey

[folex]

Most important changes are:

• fromBytes allows to pass any seed you want
• constructor takes PeerId (previously was empty and maybe private)
• toEd25519PrivateKey to complete the lifecycle.

API is as follows:

export class KeyPair {
    /**
     * Key pair in libp2p format. Used for backward compatibility with the current FluencePeer implementation
     */
    public Libp2pPeerId: PeerId;

    constructor(libp2pPeerId: PeerId) { ... }

    /**
     * Generates new KeyPair from base64 string containing the 32 byte Ed25519 private key
     * @returns - Promise with the created KeyPair
     */
    static async fromEd25519SK(base64Key: string): Promise<KeyPair> {
        // deserialize private key from base64
        const key = base64.toByteArray(base64Key);
        return await KeyPair.fromBytes(key);
    }

    /**
     * Generates new KeyPair from a 32 byte array
     * @param key - Any sequence of 32 bytes
     * @returns - Promise with the created KeyPair
     */
    static async fromBytes(arr: Uint8Array): Promise<KeyPair> { ... }

    /**
     * Generates new KeyPair with a random secret key
     * @returns - Promise with the created KeyPair
     */
    static async randomEd25519(): Promise<KeyPair> { ... }

    /**
     * @returns 32 byte private key
     */
    toEd25519PrivateKey(): Uint8Array { ... }
}