feat: add externalrole module

Add module which streamlines creation of an IAM role that can be assumed
by external entity. This is necessary for streamlining onboarding of
pollers.

modules/externalrole/README.md

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+# Observe AWS External Role
+
+This module configures an IAM role that can be assumed by Observe.
+
+## Usage
+
+```hcl
+resource "random_pet" "this" {}
+
+data "observe_cloud_info" {}
+
+data "observe_workspace" "default" {
+    name = "Default"
+}
+
+resource "observe_datastream" "example" {
+    workspace = data.observe_workspace.default.oid
+    name      = random_pet.this.id
+}
+
+module "external_role" {
+  source     = "observeinc/collection/aws//modules/externalrole"
+  name       = random_pet.this.id
+
+  observe_aws_account_id = data.observe_cloud_info.account_id
+  datastream_ids         = [observe_datastream.example.id]
+  allowed_actions        = [
+    "cloudwatch:ListMetrics",
+    "cloudwatch:GetMetricsData",
+    "tags:GetResources",
+  ]
+}
+
+```
+
+<!-- BEGINNING OF PRE-COMMIT-TERRAFORM DOCS HOOK -->
+## Requirements
+
+| Name | Version |
+|------|---------|
+| <a name="requirement_terraform"></a> [terraform](#requirement\_terraform) | >= 1.3 |
+| <a name="requirement_aws"></a> [aws](#requirement\_aws) | >= 4.0 |
+
+## Providers
+
+| Name | Version |
+|------|---------|
+| <a name="provider_aws"></a> [aws](#provider\_aws) | >= 4.0 |
+
+## Modules
+
+No modules.
+
+## Resources
+
+| Name | Type |
+|------|------|
+| [aws_iam_role.this](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/iam_role) | resource |
+
+## Inputs
+
+| Name | Description | Type | Default | Required |
+|------|-------------|------|---------|:--------:|
+| <a name="input_allowed_actions"></a> [allowed\_actions](#input\_allowed\_actions) | Set of IAM actions external entity is allowed to perform. | `list(string)` | n/a | yes |
+| <a name="input_datastream_ids"></a> [datastream\_ids](#input\_datastream\_ids) | Observe datastreams collected data is intended for. | `list(string)` | n/a | yes |
+| <a name="input_name"></a> [name](#input\_name) | Name for IAM role. | `string` | n/a | yes |
+| <a name="input_observe_aws_account_id"></a> [observe\_aws\_account\_id](#input\_observe\_aws\_account\_id) | AWS account ID for Observe tenant | `string` | n/a | yes |
+
+## Outputs
+
+| Name | Description |
+|------|-------------|
+| <a name="output_role"></a> [role](#output\_role) | IAM role to be assummed by Observe |
+<!-- END OF PRE-COMMIT-TERRAFORM DOCS HOOK -->

modules/externalrole/main.tf

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+resource "aws_iam_role" "this" {
+  name = var.name
+
+  assume_role_policy = jsonencode({
+    Version = "2012-10-17",
+    Statement = [
+      {
+        Action = "sts:AssumeRole",
+        Effect = "Allow",
+        Principal = {
+          AWS = [
+            "arn:aws:iam::${var.observe_aws_account_id}:root",
+          ]
+        }
+        Condition = {
+          StringEquals = {
+            "sts:ExternalId" = [var.datastream_ids]
+          }
+        }
+      }
+    ]
+  })
+
+  inline_policy {
+    name = "Allowed actions"
+
+    policy = jsonencode({
+      Version = "2012-10-17",
+      Statement = [
+        {
+          Action = var.allowed_actions
+
+          Effect   = "Allow",
+          Resource = "*"
+        }
+      ]
+    })
+  }
+}

modules/externalrole/outputs.tf

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+output "role" {
+  description = "IAM role to be assummed by Observe"
+  value       = aws_iam_role.this
+}

modules/externalrole/variables.tf

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+variable "name" {
+  type        = string
+  nullable    = false
+  description = <<-EOF
+    Name for IAM role.
+  EOF
+
+  validation {
+    condition     = length(var.name) <= 64
+    error_message = "Name must be under 64 characters."
+  }
+}
+
+variable "observe_aws_account_id" {
+  description = "AWS account ID for Observe tenant"
+  type        = string
+  nullable    = false
+
+  validation {
+    condition     = can(regex("^\\d{12}$", var.observe_aws_account_id))
+    error_message = "Account ID must have 12 digits."
+  }
+}
+
+variable "datastream_ids" {
+  description = <<-EOF
+    Observe datastreams collected data is intended for.
+  EOF
+  type        = list(string)
+  nullable    = false
+
+  validation {
+    condition     = length(var.datastream_ids) > 1
+    error_message = "At least one datastream must be provided."
+  }
+}
+
+variable "allowed_actions" {
+  description = <<-EOF
+    Set of IAM actions external entity is allowed to perform.
+  EOF
+  type        = list(string)
+  nullable    = false
+
+  validation {
+    condition     = length(var.allowed_actions) > 1
+    error_message = "At least one action must be provided."
+  }
+}

modules/externalrole/versions.tf

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+terraform {
+  required_version = ">= 1.3"
+  required_providers {
+    aws = {
+      source  = "hashicorp/aws"
+      version = ">= 4.0"
+    }
+  }
+}