Update s3-proxy to use env vars for all secrets

[paulbarfuss]

Changes:

• Secrets can be set with env vars (flags will still work and take precedence over env vars)
• Logger configured earlier in init
• Consistent use of logger, removed fmt.Println where possible
• Added viperBindEnv() for vars that cannot be set with the viper automatic env vars
• Implemented Stringer Interface for the Bucket object for debug logging (AccessKeys are visible, SecretKeys are obscured with "*******")
• Updated golangci-lint version and config
• Resolved linting issues

[github-actions]

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[tenyo]

I think we should update all the go dependencies since we're touching this, otherwise looks good.