feat: support estimating contract calls fees

[djordon]

Description

Closes #233.

We need the current priority mempool fees for stacks transactions. This PR adds a function that uses the POST /v2/fees/transaction endpoint on stacks-core for this.

All fees returned are the high-priority fee amount (not the fee rate). And the implementation for estimating fees in the trait falls back on the high-priority STX transfer fee in the event that the node cannot estimate fees for the transaction.

Changes

• Add a function that estimates transaction fees (for contract calls or otherwise).
• Add a function to the StacksInterface trait for estimating fees, along with a "robust" implementation that falls back to the STX transfer fee estimate if the node does not know how much the particular transaction will cost.

Testing

This PR adds an integration test that should work with both a pre- and -post-nakamoto stacks-node. It's not set up to run in CI.

[AshtonStephens]

Looks good, but I'd like a response to the "max" question I put even if we choose not to do it. Then I'll approve.

[AshtonStephens]

To move us along I'm happy to approve this ticket, but we should make a TODO note above the max part and then add a ticket. The one you just made could encompass the configuration feature as well.

[djordon]

To move us along I'm happy to approve this ticket, but we should make a TODO note above the max part and then add a ticket. The one you just made could encompass the configuration feature as well.

Done in 0092844

[djordon]

Okay, I made this more robust in #368.

[xoloki]

Lgtm. I agree that using the max fee is suboptimal, but not worth holding up this PR

[AshtonStephens]

LGTM.