Set REMOTE_ADDR in HTTP request event payload

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The Sentry event payload supports REMOTE_ADDR under the env key:

• https://develop.sentry.dev/sdk/event-payloads/request/
• https://develop.sentry.dev/sdk/event-payloads/types/#request

The :remote-addr key can be added to ring requests using ring.middleware.proxy-headers from ring/ring-headers:0.3.0 that is e.g. enabled with (ring-defaults/wrap-defaults ,,, {:proxy true}) from ring/ring-defaults:0.3.4.

If :remote-addr is not present in the request, we send the empty string and Sentry will not show this field in its web UI.