feat: support for credentials in AWS Secrets Manager

[petedannemann]

Add support for credentials stored in AWS Secrets Manager.

The Amazon Resource Name (ARN) of a secret in AWS Secrets Manager can be provided
in a cluster.yaml file as a way to provide credentials for SASL authentication. The secret in Secrets Manager must have a value in the format
shown below, identical to what AWS MSK requires.

{
  "username": "alice",
  "password": "alice-secret"
}

An example of secrets manager being used can be seen below. Be sure to include the 6Random-Characters
AWS Secrets Manager tacks on to the end of a secrets ARN.

sasl:
    enabled: true
    mechanism: SCRAM-SHA-512
    secretsManagerArn: arn:aws:secretsmanager:<Region>:<AccountId>:secret:SecretName-6RandomCharacters

Further documentation on how can be used can be seen in examples/auth/cluster.yaml or the README

[petedannemann]

I refactored NewAdminClient to take in a struct of AdminClientOpts instead of continuing our anti-pattern of passing in an ever growing list of configuration options as separate parameters. There are many instances of this change in this PR

[erushing]

Looks good to me