allow multiple users to be added to the proxy

[Guslington]

Example config

users:
  default: # disables the default user.
    disabled: true
  writer:
    secret_arn_parameter: WriterSecretCredentials
    iam_auth: REQUIRED
    description: write access to the whole database
  readonly:
    secret_arn_parameter: ReadonlySecretCredentials
    iam_auth: REQUIRED
    description: readonly access to the whole database
  audit:
    secret_arn_parameter: AuditSecretCredentials
    iam_auth: REQUIRED
    description: read and write access to the audit tables