feat: batch transactions

[ChaoticTempest]

addresses: #67.

This PR adds a new Transaction type that allows us to add in actions into it, to form a batched transaction. A lot of stuff such as client.rs can use it directly instead of making individual calls, so that it's consistent throughout, but will save that for later in another PR.

One thing to note is that batch_tx.call() takes in a CallArgs type just so we can be consistent with the account.call().args().deposit()... API, and not need to specify optional parameters such as gas/deposit. This is how it would look like when adding calls into a batch transaction:

account.batch_tx(&worker, contract_id)
    .call(CallArgs::new("function")
        .args(...)
        .deposit(0)
    )
    ...
    .transact()
    .await?;

Not sure if this API is the best, or if there's a better naming than batch_tx. Let me know if there is.

Also considered the following API for .call but probably is too funky. Also would require a trait to be exposed in the prelude:

.call("function_name".into_call_args()
    .args(...)
    .deposit(...)
)

[itegulov]

LGTM in general, just some naming nitpicking for now. I will make an attempt to use it in some of the examples later in the week and let you know my feedback.

[itegulov]

The stylistic discrepancy between these two calls looks pretty weird, but I assume this is because of rustfmt...

In general I am not a big fan of how this "reads", but at the same time I can't come up with anything better right now.

[miraclx]

lgtm.

[austinabell]

just a quick scan for now. Will dig more into it later since it seems like there can be some duplication removed, but I need to play around with it to see what is possible