TxnGroup.sign_with_private_key should raise an error if the address is not one of the senders

[fergalwalsh]

https://github.com/tinymanorg/tinyman-py-sdk/blob/40ca5536b7e31d642954ed4e3e9ee773e598c28d/tinyman/utils.py#L103

If the address is not one of the senders we should raise an error. This has tripped up some people who were passing a different address to the one used to generate the transactions but didn't get any error.

PS. Don't be tempted to refactor this to require only a private key and derive the address from that - that won't work for rekeyed accounts.