feat(identity): Add Datadog identity provider (OAuth2 + PKCE)

[srest2021]

Depends on #117026

• Adds DatadogIdentityProvider, an OAuth2 identity provider for Datadog using PKCE
• Credentials (client_id, client_secret) sourced from sentry-options (datadog.client-id, datadog.client-secret), registered as a Developer Platform OAuth app in Datadog

[linear-code]

CW-1469

[github-actions]

Backend Test Failures

Failures on 7cac1d3 in this run:

tests/sentry/identity/datadog/test_provider.py::DatadogOAuth2CallbackViewTest::test_exchange_token_success — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/identity/datadog/test_provider.py:105: in test_exchange_token_success
    assert result["access_token"] == "pkce-token"
           ^^^^^^^^^^^^^^^^^^^^^^
E   KeyError: 'access_token'

tests/sentry/identity/datadog/test_provider.py::DatadogOAuth2CallbackViewTest::test_exchange_token_no_code_verifier — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/identity/datadog/test_provider.py:120: in test_exchange_token_no_code_verifier
    assert result["error"] == "pkce_missing"
E   AssertionError: assert 'Missing pipeline state' == 'pkce_missing'
E     
E     �[0m�[91m- pkce_missing�[39;49;00m�[90m�[39;49;00m
E     �[92m+ Missing pipeline state�[39;49;00m�[90m�[39;49;00m

tests/sentry/identity/datadog/test_provider.py::DatadogOAuth2LoginViewTest::test_preserves_standard_oauth_params — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/identity/datadog/test_provider.py:55: in test_preserves_standard_oauth_params
    response = self.view.dispatch(self.request, pipeline)
               ^^^^^^^^^
/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/functools.py:1039: in __get__
    val = self.func(instance)
          ^^^^^^^^^^^^^^^^^^^
tests/sentry/identity/datadog/test_provider.py:38: in view
    return DatadogOAuth2LoginView(
E   TypeError: DatadogOAuth2LoginView.__init__() got an unexpected keyword argument 'client_id'

tests/sentry/identity/datadog/test_provider.py::DatadogOAuth2CallbackViewTest::test_login_to_callback_preserves_code_verifier — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/identity/datadog/test_provider.py:135: in test_login_to_callback_preserves_code_verifier
    login_view = DatadogOAuth2LoginView(
E   TypeError: DatadogOAuth2LoginView.__init__() got an unexpected keyword argument 'client_id'

tests/sentry/identity/datadog/test_provider.py::DatadogOAuth2LoginViewTest::test_includes_pkce_params — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/identity/datadog/test_provider.py:44: in test_includes_pkce_params
    response = self.view.dispatch(self.request, pipeline)
               ^^^^^^^^^
/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/functools.py:1039: in __get__
    val = self.func(instance)
          ^^^^^^^^^^^^^^^^^^^
tests/sentry/identity/datadog/test_provider.py:38: in view
    return DatadogOAuth2LoginView(
E   TypeError: DatadogOAuth2LoginView.__init__() got an unexpected keyword argument 'client_id'