Add lastUpdateTime to aircraft VarBC files

[CoryMartin-NOAA]

Description

This adds lastUpdateTime as an int64 seconds since 1970 that IODA can decipher (hopefully!)

Issue(s) addressed

Resolves #

Dependencies

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Impact

Expected impact on downstream repositories:

Checklist

• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have run the unit tests before creating the PR

[BenjaminRuston]

ahh,, this looks reasonable but when I tested I got failure on:

iodaconv_validate_testoutput_acft_bias.20230913T00Z.nc

will circle back to this again as I must've crossed something up