feature(logger): add support for creating new logger

[squizzi]

InitLog has been renamed to New and InitLog now calls New, this allows us to get a localized new logger outside of the default Logger.

I tested this against PR: https://github.com/replicatedhq/vandoor/pull/6859

The eventLog logger instantiated with log.New fires in JSONFormatter but the other logger fires everything else:

{"caller":"cluster/event_log.go:49","cluster_guid":"7e95da15","cluster_name":"serene_wright","distribution":"fake","instance_types":["r1.small"],"level":"info","message":"cluster create resulted in error status","node_count":1,"node_groups":1,"status":"error","team_id":"2rMLsxkHYFV27t5oCfSyYOfVtNH","team_name":"Mango, Inc.","team_url":"https://admin.replbiz.com/teams/2rMLsxkHYFV27t5oCfSyYOfVtNH","timestamp":"2025/02/06 04:47:16","ttl":"1h","version":"1.37"}
[GIN] 2025/02/06 - 04:47:16 | 200 |   98.238281ms | 10.0.6.135 |   PUT     /internal/v3/reliability_matrix/cluster_provisioner
DEBUG 2025/02/06 04:47:16 /go/pkg/mod/github.com/replicatedcom/saaskit@v0.0.0-20250206043459-1b1d5ef61e42/log/gin.go:45 [GIN] 200 |   98.04784ms | 10.0.6.135 | PUT     /internal/v3/reliability_matrix/cluster_provisioner
ERROR 2025/02/06 04:47:16 cluster/cluster.go:2025 Failed to send slack notification for cluster 7e95da15: failed to update cluster event slack message: channel_not_found
[GIN] 2025/02/06 - 04:47:16 | 200 |   20.863855ms | 10.0.6.135 |   PUT     /internal/v3/reliability_matrix/cluster_provisioner_versions