[admin][db] project retention policy based on number of spans

[mikeldking]

A project by default should come with a retention policy of X number of spans. This means as new traces are ingested, spans would fall off the tail of the project to minimize the database size "growing" indefinitely. This should be configurable per project.

[RogerHYang]

We should probably delete whole traces instead, otherwise the trace tree could be fragmented with orphaned spans.

[BenMcH]

Can a timeframe be considered in addition to a span count? There are a number of industries that have minimum retention requirements that this would help with.

IE: purge only after X years