Add extended keyword to ignore "length" keyword in metadata and declare variable a scalar

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

_No response_

### Brief description

MetOffice/jules/issues/41 migrated `namelist:jules_pftparm` metadata to **jules-shared/jules-pftparm** and implemented the duplicate namelist feature to allow each PFT to have its own namelist e.g. `namelist:jules_pftparm(brd_leaf)`. The aim is to have one source of JULES metadata **jules-shared** which is imported into the parent model:

- **um-atmos** (MetOffice/um) imports **jules-um**
- **jules-lsm** (MetOffice/lfric_apps) imports **jules-lfric**

**jules-shared** is imported by **jules-standalone**, **jules-um** & **jules-lfric**, which add the parent amendments or parent specific metadata. It is desirable to have as little as possible in the parent specific metadata and the majority of the metadata in **jules-shared**. UM & JULES-standalone does not have the duplicate namelist functionality and as such `jules_pftparm` variables are arrays and require the `length=:` keyword. However, the presence of length keyword in the **jules-shared** metadata causes the configurator to declare the the variable as an array, which causes problems when trying to assign the value to `pftparm_mod`. This could be dealt with in the code but not in an elegant way. More satisfactory would be for the configurator to generate the appropriate scalar code, which can be achieved by adding an extended keyword (suggested by @mo-rickywong).

### Further details of the issue.

_No response_