`EvaluateTx` function returns an empty slice due to JSON field tags mismatch

[MIxAxIM]

Describe the bug
The ExecutionUnits struct has JSON field tags with incorrect capitalization. The JSON response received contains lowercase keys ('mem' and 'steps'), while the struct expects 'Mem' and 'Steps', causing a mismatch. As a result, the EvaluateTx function returns an empty slice. Additionally, replacing []models.RedeemerEvaluation with models.EvaluateTxResponse in the EvaluateTx function improves consistency and readability.

To Reproduce

Steps to reproduce the behavior:

1. Try EvaluateTx function
2. The example response is

[{"redeemer_tag":"mint","redeemer_index":0,"ex_units":{"mem":745242,"steps":186570312}}]

3. Unmarshaling doesn't work
4. Go to models/transactions.go
5. check ExecutionUnits struct has a different JSON tag