Add UpdateMixnodeConfigOnBehalf to vestng contract

[durch]

No description provided.

[tommyv1987]

Happy with this.
Steps to test:
Migrate both vesting and mixnet contracts.
Have an existing account with a node bonded and perform the following:

 let mut rng = rand::thread_rng();
        let profit_percent: u8 = rng.gen_range(1..100);

        let response = client.vesting_update_mixnode_config(profit_percent)
            .await
            .unwrap();

        println!("{:?}", response);

Then, get the account details and the mixnode profit percentage should be updated with the value set above:

Some(MixNodeBond { pledge_amount: Coin { denom: "unymt", amount: Uint128(100000000) }, total_delegation: Coin { denom: "unymt", amount: Uint128(0) }, owner: Addr("address"), layer: One, block_height: 442421, mix_node: MixNode { host: "1.1.1.1", mix_port: 1789, verloc_port: 1790, http_api_port: 8000, sphinx_key: "key", identity_key: "key", version: "0.12.1", profit_margin_percent: 99 }, proxy: Some(Addr("address")) })

[jstuczyn]

Should we perhaps futureproof ourselves a bit and make it take Option and Option to let you update both?

[durch]

Should we perhaps futureproof ourselves a bit and make it take Option and Option to let you update both?

I'll merge it like this for now, its been dangling a while