Use application-specific useragent string for outgoing http requests

[baszoetekouw]

Use a application-specific useragent string instead of Java/21.0.10.

Include:

• the application name
• the application version
• any user-specific strings passed via the environment variable USER_AGENT_EXTRA (may be empty)

for example if USER_AGENT_EXTRA=prod/docker1rood the user-agent string becomes attribute-aggregation/1.2.3/prod/docker1rood.