Allow appending caller-supplied data to the User-Agent (pip's PIP_USER_AGENT_USER_DATA equivalent)

Issue Kind

Brand new capability

Description

pip lets us set PIP_USER_AGENT_USER_DATA, which it embeds as user_data in its User-Agent. We use it to add extra information to our requests. It would be important for us to have an equivalent in poetry (measured on 2.3.4, the User-Agent is fixed: poetry/2.3.4 CPython/3.14.4 Darwin/25.6.0). Is there anything compatible we can use today?

Impact

Requests made by poetry can't carry caller context the way pip's can.

Workarounds

None found, no environment variable or config alters the User-Agent.