fix(iam): added username and password for iam accesskey connection-secret

[haarchri]

Description of your changes

add username and password for iam accesskey connection secret like community provider-aws

community provider-aws:

apiVersion: v1
data:
  password: *****
  username: *****

official provider-aws:

apiVersion: v1
data:
  attribute.secret: *****
  attribute.ses_smtp_password_v4: *****

after that PR we have 2 additional keys for official provider-aws:

apiVersion: v1
data:
  attribute.secret: *****
  attribute.ses_smtp_password_v4: *****
  password: *****
  username: *****

think today its not possible to run an uptest action for this AdditionalConnectionDetailsFn we have one open issue for that: upbound/official-providers-ci#82

Fixes #479

I have:

• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

create user.yaml and checked secret

NAME                                  READY   SYNCED   EXTERNAL-NAME   AGE
user.iam.aws.upbound.io/sample-user   True    True     sample-user     84s

NAME                                             READY   SYNCED   EXTERNAL-NAME          AGE
accesskey.iam.aws.upbound.io/sample-access-key   True    True     AKIAWLLR7LGH24LYMW5R   84s

NAME                       TYPE                                DATA   AGE
sample-access-key-secret   connection.crossplane.io/v1alpha1   4      85s

[sergenyalcin]

Thanks @haarchri LGTM!