Send individual diagnostic fields via lfric-xios interface

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Currently the LFRic-XIOS interface I/O implementation is purely file-based. All the I/O operations that can be scheduled in a run can only be configured to run on whole files. This restricts many of the interesting and dynamic operations that can be done on a field-by-field basis, where fields can be sent to XIOS and then processed ad diagnostics on the XIOS servers before being written to files.

This issue will create an interface which will enable the sending of fields to XIOS as diagnostics, with a similar time-scheduling system as the `lfric_xios_file_type`

### Further details of the issue.

_No response_