feat(seer-infra-telemetry): Add Datadog PAT connect flow to monitoring providers UI

[shashjar]

Resolves CW-1542.

See the Seer Advanced Settings page for more.

Adds a modal to the monitoring providers UI for connecting Datadog via Personal Access Token (PAT), as an alternative to OAuth. The PAT modal collects the access token (password-masked) and the Datadog site (via a selection from the list of valid/possible sites). On connection creation, it POSTs {access_token, site} to the existing backend endpoint, which returns 204. The disconnect flow works identically for both OAuth providers and PAT providers.

Gated behind the organization seer-infra-telemetry feature flag.

Main page

After attempting to connect with an invalid token or incorrect site

Main page after successful provider connection

Confirmation modal for disconnecting a provider

[linear-code]

CW-1542

[srest2021]

Should we feature flag this?

[shashjar]

@srest2021 the monitoring providers section is already feature flagged (so we won't display that section on Seer Advanced Settings without the flag) -

sentry/static/gsApp/views/seerAutomation/advanced.tsx

Lines 90 to 92 in 72396f9

	{organization.features.includes('seer-infra-telemetry') && (
	<MonitoringProvidersSection />
	)}

[srest2021]

Lgtm for internal testing