Ability to reload the regex configuration file used for device type categorisation

[paulb-elastic]

This recent PR added the capability to categorise the device type from the UA string. As we mature the categorisation and new devices come to market, there will be a need to update the configuration that the user agent processor uses to categorise the device type.

Although each new stack release may include an updated category file, it would be useful to include the ability for users to update this configuration without having to upgrade their whole stack (much like they can do today with the traditional user agent parser file).

ACs:

• A user can update the configuration file that is used by the user agent parser responsible for categorising the device type (independently of a stack upgrade)
• The user may have to reload the ingest pipeline for the updated file to take effect
• This process is documented (for example, a user can take an updated version of the configuration file committed in the repo and apply it to their existing stack that supports device type categorisation)

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)