Workload ID: Support templating/rules using User/Bot traits

[strideynet]

Today, Workload Identity supports a wide range of attributes for templating, however, it does not support using the traits of the Bot or User who is requesting the issuance of the identity.

Traits are particularly interesting since this is more complex than map[string]string - we'll need to consider how users would likely template in cases where multiple trait values are provided. We may want to accept a simpler case of rejecting templating where multiple values are provided.

Likely blocked on #49985