feature: add support for experimental investigation feature

[matryer]

Explore Metrics and Explore Logs have introduced an experimental feature for collecting interesting signals together during an investigation. We would like to add support for Traces:

• decide data model for v1-alpha API (work with @fcjack)
• add UI support (same three dots menu) to allow people to add items to an investigation (see this on internal Slack)
• add time series panels
• add a trace

[joey-grafana]

Marc has listed the steps he took with explore profiles here.