support backoff between retries in udp_proxy

[IssaAbuKalbein]

Commit Message: support backoff between retries in udp_proxy
Additional Description: This feature also fixes a bug in the retry mechanism in udp proxy in the following situation:

1. We are tunneling UDP over HTTP.
2. A new stream is created on existing upstream connection (multiplexing) and waiting for response headers.
3. The upstream connection is closed.
4. During the close process, all streams created on this connection will be reset.
5. The udp_proxy receives a callback on the stream reset.
6. They retry to connect.
7. The closed connection is picked as it is still in the connection pool (we are still in the process of the close).
8. The new stream that is created on the second attempt will be reset immediately.
9. The same process will happen (step 5 - step 8) until we reach the max_connect_attepts.
10. This means that we are doing only one real attempt in this situation.

Risk Level: medium
Testing: unit tests, integration tests
Docs Changes: added
Release Notes: added

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #37912 was opened by IssaAbuKalbein.

see: more, trace.

[IssaAbuKalbein]

/retest