[Feature]: support specifying the number of times an event was emitted

[0xNeshi]

What is the feature you would like to see?

The proposed syntax is:

contract.assert_emitted(event: Event).times(number_of_times: u32);

Possible values for number_of_times:

• 0 - assert that the event was never emitted <-- (optional) could create a new extension like contract.assert_never_emitted that does the same
• 1+ - assert that event was emitted this exact number of times.

To give a dumb, and yet possible example where this would be useful would be a Vault that can execute multiple pending withdrawal requests in one transaction, collecting fees for the Vault owner and emitting something like FeeCollected for each withdrawal.

#[test]
fn multiple_withdrawals_work(
    owner: Account,
    alice: Account,
    bob: Account
) {
    let vault = Contract::<Vault>::new();

    // set up the vault

    // alice and bob invest in the vault
    
    // alice and bob initiate withdrawal requests
    
    vault.withdraw_pending().expect("ok");
    
    // Assert emitted exactly twice with macro
    assert_emitted!(vault, FeeCollected { }, 2);
    // or as a contract extension
    vault.assert_emitted(FeeCollected { }).times(2);
}

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