Add memo when creating a vault

[dannpl]

We going to create a open source socialFi for slide.

We want to add a flag to find easy if the vault is created by Slide

https://github.com/slide-labs/slide-socialfi

it's helpfully if add a memo

drift-vaults/programs/drift_vaults/src/state/vault.rs

Line 28 in 76c2751

pub struct Vault {

#[assert_no_slop]
#[account(zero_copy)]
#[derive(Default, Eq, PartialEq, Debug)]
#[repr(C)]
pub struct Vault {
    /// The memo of the vault. (e. g Slide - Vault)
    pub memo: [u8; 32],

[0xbigz]

using prefix/suffix for names can achieve a similar thing