Add batch transaction in fee estimator

### Background

HIP-551 introduced batch transactions. While the prizes for inner transactions did not change, we should add an entry to calculate the fees for an empty batch transaction. If possible, the site should show a note that the fees for inner transactions have to be calculated separately.

### Acceptance Criteria

1. The fee estimator allows the user to calculate the fees for a batch transaction.

### Dependencies

_No response_

### Definition of Ready (DoR) Checklist

- [ ] Clear acceptance criteria
- [ ] Clear and detailed description
- [ ] Dependencies identified
- [ ] Links to documentation
- [ ] Should be completable in 2-3 Days
- [ ] Initial draft of Low-level design document
- [ ] At least high level test plan
- [ ] Groomed/Estimated

### Definition of Done (DoD) Checklist

- [ ] Acceptance Criteria complete
- [ ] No Codacy issues greater than minor (in new code)
- [ ] JavaDocs updated/created
- [ ] Code commented
- [ ] Unit tests created/updated
- [ ] 80% test code coverage (in new code)
- [ ] Happy Path and major negative cases in HAPI tests as applicable

No files, tests, or entry points are named. Start by locating the fee-estimator transaction entries and existing fee-calculation tests, then verify that an empty batch transaction can be estimated and that any note about calculating inner-transaction fees separately is covered.

Written by the indexing model from the issue text.