feat: Add company model and API with tests (#12548)

# Pull Request Template

## Description

* add Company model with validations for name, domain, description and
  avatar
* Add database migration fo
* Implement endpoints for company CRUD operations
* Add optional company relationship for contacts
* Add test for models, controllers, factories and policies
* Add authorization policies restricting delete to admins
* support JSON API responses
Please include a summary of the change and issue(s) fixed. Also, mention
relevant motivation, context, and any dependencies that this change
requires.

Fixes #(cw-5650)

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

Tests are implemented using `RSpec`

```
$ bundle exec rails db:migrate
$ bundle exec rspec spec/models/company_spec.rb spec/controllers/api/v1/accounts/companies_controller_spec.rb
```

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

spec/enterprise/policies/company_policy_spec.rb

@@ -0,0 +1,33 @@
+require 'rails_helper'
+
+RSpec.describe CompanyPolicy, type: :policy do
+  subject(:company_policy) { described_class }
+
+  let(:account) { create(:account) }
+  let(:administrator) { create(:user, :administrator, account: account) }
+  let(:agent) { create(:user, account: account) }
+  let(:company) { create(:company, account: account) }
+
+  let(:administrator_context) { { user: administrator, account: account, account_user: account.account_users.first } }
+  let(:agent_context) { { user: agent, account: account, account_user: account.account_users.first } }
+
+  permissions :index?, :show?, :create?, :update? do
+    context 'when administrator' do
+      it { expect(company_policy).to permit(administrator_context, company) }
+    end
+
+    context 'when agent' do
+      it { expect(company_policy).to permit(agent_context, company) }
+    end
+  end
+
+  permissions :destroy? do
+    context 'when administrator' do
+      it { expect(company_policy).to permit(administrator_context, company) }
+    end
+
+    context 'when agent' do
+      it { expect(company_policy).not_to permit(agent_context, company) }
+    end
+  end
+end