add support for fetching values from key/value secrets (AWS Secrets Manager only)

[cheddarwhizzy]

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Currently this repo only supports a single plaintext secret to be fetched. This PR allows you to specify the same ARN with a #KEY to fetch only the value from that key. This was inspired from previously using https://github.com/banzaicloud/bank-vaults/tree/master/charts/vault-secrets-webhook

- name: PGPASSWORD
  value: arn:aws:secretsmanager:us-west-2:<account_id>:secret:my-database-secret#DB_PASSWORD
- name: PGUSER
  value: arn:aws:secretsmanager:us-west-2:<account_id>:secret:my-database-secret#DB_USER

EDIT: Testing this requires adding 2 flags to kube-secrets-init command

- --image=yourrepo/this-image:latest
- --pull-policy=Always

[alexei-led]

@cheddarwhizzy would you like to rebase this PR and decide either to enhance existing functionality