If the FDB::retrieve output DataHandle is not seekable, add the ability to buffer in memory 1 output message, to enable seek backward within the cached message.

### Is your feature request related to a problem? Please describe.

ability to decode the GRIB message headers (with support for both GRIB 1 and 2)

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Organisation

_No response_