[APM] Add user.agent.name and user.agent.version to the trace header summary

[formgeist]

Summary

Related to #46078

In #44842 we implemented a new design for displaying the summary of information on the trace in the header. In that process we removed the user.agent information which was added in a previous release.

Tasks

We want to add pieces of that information back into the summary, more specifically;

• user.agent.name
• user.agent.version

The rest of the user.agent information can be found in the metadata tab.

This addition needs to also be implemented in the following components;

• Transaction details flyout header
• Error occurrence header

[elasticmachine]

Pinging @elastic/apm-ui

[sorenlouv]

Closed by #47526