fix: external redis functionality

[iamyogeshg]

This PR extends on to the feature by #1492

We are using external redis from aws elasticache in non cluster mode.

externalRedis:
  host: master.uat-XXXXXX.amazonaws.com
  existingSecret: sentry
  existingSecretKey: redis-password
  ssl: true

redis:
  enabled: false

With above config we were facing this issue on sentry relay

2024-10-14T04:00:01.703356Z ERROR relay_server::services::processor: error processing envelope tags.project_key=834fe1715ec71e6042077d96dbde5ca4 error=failed to apply quotas error.sources=[failed to communicate with redis, failed to communicate with redis, NOAUTH: Authentication required.]

As per this issue, we have to provide username:password format and if not, we should precede the password with the colon which was unintentionally missed out for relay service. Please note we did have the colon here in the same PR.