[FEATURE] Update to Elastic Common Schema 1.8

[russcam]

Core minimum for release 1.8.0

• Autogenerate from the schema definition.
• Fix up any compatibility issues with clients/usage, e.g. different loggers.

Depending on the extent of changes, for anything new can either be added as part of the main release, or added as subsequent point releases.

• Review schema changes/additions for anything new that is worth adding to Extensions
• Review schema changes/additions for anything new that is worth adding to NLog
• Review schema changes/additions for anything new that is worth adding to Serilog
• Review schema changes/additions for anything new that is worth adding to other libraries

These don't all have to be done at once, or as a block, i.e. can incrementally add to different libraries as time permits.