feat(seer-infra-telemetry): Add monitoring providers section to Seer Advanced Settings page

[shashjar]

Resolves https://linear.app/getsentry/issue/CW-1500/monitoring-providers-settings-page.

Adds a (temporary, for testing) monitoring providers UI section to the Seer Advanced Settings page, gated behind the organizations:seer-infra-telemetry feature flag. This lets users connect and disconnect infrastructure monitoring providers (Datadog, GCP) via the API endpoints previously added in #117854.

• Lists available providers with connection status from GET /monitoring-providers/
• Connect button initiates the OAuth flow via POST /monitoring-providers/{provider}/ and redirects to the provider's authorize page
• Disconnect button shows a confirmation modal, then calls DELETE /monitoring-providers/{provider}/ and refreshes the list

Note that the Datadog logic hardcodes site: "datadoghq.com" for v0 (https://linear.app/getsentry/issue/CW-1501/support-per-site-datadog-connections-in-monitoring-provider-endpoints tracks per-site support for multiple connected Datadog orgs for a single user).

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[cursor]

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[linear-code]

CW-1500

[wedamija]

lgtm as a temporary page