Open XIOS context definition

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

_No response_

### Brief description

We aim to make the configuration of XIOS contexts within LFRic more accessible by decoupling the context definition from the context initialisation.

### Further details of the issue.

In the current LFRic-XIOS implementation, the definition of the XIOS context is performed during the initialisation of the `lfric_xios_context_type` object. This means that a large amount of complex configuration work needs to happen in the space of a single routine, which has lead to the introduction of procedure pointers and functors in order to make sure everything happens at the correct time. Currently, everything is passed to the `init_io` subroutine at the driver level, which does not adequately allow for configuration of models with increasing I/O complexity.
If we can successfully decouple the XIOS context _definition_ from the initialisation of the `lfric_xios_context_type`, then we can potentially allow for a longer period in which the XIOS context can be defined, which will allow for greater flexibility for our I/O capability.

Previous work was done on a trac branch - here: https://code.metoffice.gov.uk/trac/lfric/log/LFRic/branches/dev/edwardhone/r53806_openIoDefinition

Tagging: @andrewcoughtrie @stevemullerworth