Hex data field on send flow needs validation

Describe the bug
If the "Show Hex Data" advanced setting is enabled, the "Hex data" field can be filled with any input and will be included in the transaction.

Steps to reproduce (REQUIRED)

1. Enable "Show Hex Data" in the advanced settings
2. Initiate a send
3. Enter `test` or other non-sensical input in the "Hex data" field
4. Click next
5. See that that confirmation shows a non-zero bytes amount.

Expected Behavior
the hex data field doesn't allow non hex string input.

Start from the send flow and reproduce the issue with the advanced “Show Hex Data” setting enabled, entering `test` in the “Hex data” field. Trace the field through the Next step and confirmation behavior. Done means non-hex input is rejected and cannot contribute to the transaction’s data or byte count.

Written by the indexing model from the issue text.