Add estimateMinFee function to UnsignedTx

[palas]

Changelog

- description: |
    Add `estimateMinFee` function to `UnsignedTx` in `cardano-wasm` API
  type:
  - feature
  projects:
  - cardano-wasm

Context

This build upon cardano-wasm creation PRs:

• Add minimal API for creating transactions #893
• Add JS wrapper and HTML example for wasm API #894

It simply adds an extra functionality to estimate fees for a transaction.

How to trust this PR

The best way would be testing. But the API itself it is a thin wrapper. And the example changed very subtly. I think the main thing to check is the style of the code and documentation.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff