New Resource: aws_fis_target_account_configuration

[RezaImany]

Summary

Adds aws_fis_target_account_configuration to manage FIS target account configuration for multi‑account experiment templates. This wraps the AWS FIS CreateTargetAccountConfiguration API.
Motivation

FIS supports multi-account experiments where the template runs actions in additional AWS accounts. This resource registers a target account and the role FIS assumes to execute the experiment there.
Implementation

Resource: internal/service/fis/fis_target_account_configuration.go
Create: calls CreateTargetAccountConfiguration with ClientToken = id.UniqueId() for idempotency; sets composite ID <experiment_template_id>/<account_id>.
Read: calls GetTargetAccountConfiguration, updates state; clears state on ResourceNotFoundException.
Update: if description changes, reissues CreateTargetAccountConfiguration with the same IDs (idempotent in AWS) and new description; re-reads state.
Delete: calls DeleteTargetAccountConfiguration, tolerates already-deleted ResourceNotFoundException.
Import: supports composite ID <experiment_template_id>/<account_id>.
Timeouts: create/update/delete default 30m.
Registration: internal/service/fis/service_package_gen.go adds the resource factory for aws_fis_target_account_configuration.
Finder/waiters: helper functions use composite ID parsing and the FIS SDK v2 types.TargetAccountConfiguration.
Removed scaffold-only helpers; code uses only required logic.
Schema

account_id (Required, ForceNew)
experiment_template_id (Required, ForceNew)
role_arn (Required, ForceNew)
description (Optional)
Identity and Import

ID is <experiment_template_id>/<account_id>.
Import accepts the same composite ID.
Behavior Notes

Only required when experiment_options.account_targeting in the template is multi-account.
Update: only description is mutable; implemented via idempotent CreateTargetAccountConfiguration call (no dedicated Update API).
Not Found: ResourceNotFoundException removes the resource from state during Read and is ignored during Delete.
Tests

internal/service/fis/fis_target_account_configuration_test.go
Basic: provisions an experiment template + target account config, asserts fields, and verifies import.
Disappears: deletes target account configuration out of band and expects recreation on plan.
Description update: updates description and verifies state.
Exports: internal/service/fis/exports_test.go exposes FindTargetAccountConfigurationByID for tests.
Documentation

Canonical: website/docs/r/fis_target_account_configuration.html.markdown
Parallel: website/docs/r/fis_target_account_configuration.md
Legacy stub: website/docs/r/fis_fis_target_account_configuration.html.markdown (redirects readers to the new page)
Includes example usage, argument/attribute docs, import, timeouts, and a link to the AWS API docs.
Generation

Ran make gen to register resource and update generated files.
Manual Validation

Built provider locally and validated end-to-end using a sample Terraform config:
terraform init && terraform apply to create, then delete via console/CLI and re-plan to confirm recreation, then terraform destroy.
Checklist

New resource
Acceptance tests (basic, disappears, argument update)
Documentation
make gen, make fmt, make lint (note: local linter may require pinned Go version)
Unit tests pass (make test)
No breaking changes

Closes #44884

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