--aws-iam-idp and --aws-iam-role alias values

[monde]

--aws-iam-idp and --aws-iam-role values can be friendly label and we'll reverse look up the ARN value for them from okta.yaml.

Closes #102

Given an okta.yaml of

---
awscli:
  idps:
    "arn:aws:iam::123:saml-provider/S3_IDP": "S3 IdP"
    "arn:aws:iam::123:saml-provider/EC2_IDP": "EC2 IdP"
  roles:
    "arn:aws:iam::123:role/S3_RW": "S3 Read/Write"
    "arn:aws:iam::123:role/S3_READ": "OK S3 Read"
    "arn:aws:iam::123:role/EC2_full_access": "EC2 FULL"

okta-aws-cli can be invoked with ARN or friendly label values:

# these two commands are equivalent

$  okta-aws-cli web --oidc-client-id 0oabc --org-domain test.okta.com \
      --aws-iam-idp arn:aws:iam::123:saml-provider/S3_IDP \
      --aws-iam-role arn:aws:iam::123:role/S3_READ

$  okta-aws-cli  web --oidc-client-id 0oabc --org-domain test.okta.com \
      --aws-iam-idp "S3 IdP" \
      --aws-iam-role "OK S3 Read"

[monde]

addressed in https://github.com/okta/okta-aws-cli/releases/tag/v2.1.0