feat(contract): return multicall pending transaction

[DaniPopes]

Motivation

2 year-old TODO comment from #107

TODO: Can we make this return a PendingTransaction directly instead?
Seems hard due to `returns a value referencing data owned by the current function`

The only data that PendingTransaction has to hold a reference to is M::Provider. which we can get with Arc::as_ref but this is currently not exposed

Solution

add client_ref method that returns Arc::as_ref(self.client) for Contract
return PendingTransaction in Multicall::send

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog
• Breaking changes

[mattsse]

lgtm, but this is a breaking change and needs to be added to the changelog

I don't have strong feelings about this, perhaps a separate function that awaits the pedingtx would be useful?