Add u__ prefix to secret names in propeller (#302)

## Overview
Adds `u__` prefix to secrets in propeller.

This change corresponds to step 3 in rollout plan below.

## Test Plan
1. Deploy dogfood managed cluster.
2. Create secret.
3. Run a task referencing that secret.

## Rollout Plan
1. Copy secrets into `u__` prefix (now we will have secrets with both `org__` and `u__` prefixes.)
2. Deploy dataproxy in control plane.
4. Deploy data planes. (This change.)
5. Delete secrets with `org__` prefix.

There will be some downtime between steps 2 and 3 for secret operations through CLI.

## Upstream Changes
Should this change be upstreamed to OSS (flyteorg/flyte)? If so, please check this box for auditing. Note, this is the responsibility of each developer. See [this guide](https://unionai.atlassian.net/wiki/spaces/ENG/pages/447610883/Flyte+-+Union+Cloud+Development+Runbook/#When-are-versions-updated%3F).
- [ ] To be upstreamed

## Issue
CLD-956

## Checklist
* [ ] Added tests
* [ ] Ran a deploy dry run and shared the terraform plan
* [ ] Added logging and metrics
* [ ] Updated [dashboards](https://unionai.grafana.net/dashboards) and [alerts](https://unionai.grafana.net/alerting/list)
* [ ] Updated documentation

flytepropeller/pkg/webhook/embedded_secret_manager.go

@@ -20,7 +20,8 @@ const (
 	UnionSecretEnvVarPrefix                                     = "_UNION_"
 	SecretFieldSeparator                                        = "__"
 	ValueFormatter                                              = "%s"
-	SecretsStorageOrgPrefixFormat                               = "org" + SecretFieldSeparator + ValueFormatter
+	SecretsStorageUnionPrefix                                   = "u"
+	SecretsStorageOrgPrefixFormat                               = SecretsStorageUnionPrefix + SecretFieldSeparator + "org" + SecretFieldSeparator + ValueFormatter
 	SecretsStorageDomainPrefixFormat                            = SecretsStorageOrgPrefixFormat + SecretFieldSeparator + "domain" + SecretFieldSeparator + ValueFormatter
 	SecretsStorageProjectPrefixFormat                           = SecretsStorageDomainPrefixFormat + SecretFieldSeparator + "project" + SecretFieldSeparator + ValueFormatter
 	SecretsStorageFormat                                        = SecretsStorageProjectPrefixFormat + SecretFieldSeparator + "key" + SecretFieldSeparator + ValueFormatter