fix: decouple IAM policy documents from IAM role resource

[cazorla19]

Description

Fixes #193

additionally decouples assume_role_policy from IAM role specifications to the separate data.aws_iam_policy_document resource

How Has This Been Tested?

0. Run the module instance via terraform apply before the code change to reproduce this warning:

│ Warning: Argument is deprecated
│ 
│   with module.cloudwatch_firehose_logs_coralogix.aws_iam_role.new_firehose_iam[0],
│   on .terraform/modules/cloudwatch_firehose_logs_coralogix/modules/firehose-logs/main.tf line 97, in resource "aws_iam_role" "new_firehose_iam":
│   97: resource "aws_iam_role" "new_firehose_iam" {
│ 
│ The inline_policy argument is deprecated. Use the aws_iam_role_policy resource instead. If Terraform should exclusively manage all inline policy associations (the current behavior of this argument), use the aws_iam_role_policies_exclusive resource as well.

1. Run terraform plan on firehose-logs module instance after the code change
2. You will see a new resource Terraform wants to add, run terraform apply to see the warning is gone and the new resource has been added without breaking the resource permission

Although terraform will technically create a new resource, practically the state of IAM role altogether with its policy will stay the same as it was before the change: same inline policy format, same name, same permissions. (during tf apply it won't return you the error that this policy already exists)

Checklist:

• I have updated the relevant example in the examples directory for the module.
• I have updated the relevant test file for the module.
• This change does not affect module (e.g. it's readme file change)

[daidokoro]

LGTM

[github-actions]

🎉 This PR is included in version 2.3.2 🎉

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