Add upstream proxy feature

[eraow]

Note: This PR was generated with AI assistance.

Summary

Adds an --upstream-proxy option so Boundary can forward its own outbound
requests through an HTTP(S) proxy. This makes Boundary usable in environments
that only allow internet access via a corporate proxy.

Motivation

I want to run Boundary behind a corporate proxy. Without this option, Boundary
connects directly to upstream hosts, which fails when direct egress is blocked
and only a proxy is permitted.

What this does

• Adds --upstream-proxy <URL> (env BOUNDARY_UPSTREAM_PROXY, YAML
  upstream_proxy), e.g. http://proxy.corp:3128.
• When set, Boundary routes its own upstream HTTP/HTTPS requests through the
  given proxy instead of connecting directly. Allow-rule evaluation and audit
  logging still happen first — the upstream proxy only receives already-allowed
  requests.
• Behavior is unchanged when the flag is not provided.
• The upstream proxy is a Boundary outbound-transport setting, not a child
  HTTP_PROXY setting. The confined child still sends traffic to Boundary first.

Usage

boundary --upstream-proxy http://proxy.corp:3128 \
  --allow "domain=github.com" -- curl https://github.com

Manual verification

PASS (allowed domain)

$ ./boundary --upstream-proxy http://${proxy}:3080 --log-level info --log-dir ./boundary-logs --allow "domain=github.com" -- curl -s -o /dev/null -w "%{http_code}" https://github.com
200
$ cat  boundary-logs/boundary-2026-07-17_00-08-01-4094015.log
time=2026-07-17T00:08:01.237+09:00 level=INFO msg="boundary session started" session_id=09d4a8ab-7ba4-42ae-a90f-9ff79e37c608
time=2026-07-17T00:08:01.237+09:00 level=WARN msg="Audit logs are disabled; workspace agent has not created log proxy socket" socket=/tmp/boundary-audit.sock
time=2026-07-17T00:08:01.237+09:00 level=INFO msg="Using upstream proxy" upstream_proxy=http://proxy:3080
time=2026-07-17T00:08:01.237+09:00 level=INFO msg="Start namespace-jail manager"
time=2026-07-17T00:08:01.246+09:00 level=INFO msg="Starting HTTP proxy with TLS termination" port=8080
time=2026-07-17T00:08:01.398+09:00 level=INFO msg=ALLOW method=GET url=https://github.com/ host=github.com rule="domain=github.com"
time=2026-07-17T00:08:01.473+09:00 level=INFO msg="Command completed, shutting down..."
time=2026-07-17T00:08:01.473+09:00 level=INFO msg="Stop namespace-jail manager"

BLOCK (disallowed domain — never reaches the upstream proxy)

$ ./boundary --upstream-proxy http://${proxy}:3080 --log-level info --log-dir ./boundary-logs --allow "domain=github.com" -- curl -s -o /dev/null -w "%{http_code}" https://example.com
403
$ cat  boundary-logs/boundary-2026-07-16_23-11-43-3997580.log
time=2026-07-16T23:11:43.132+09:00 level=INFO msg="boundary session started" session_id=f11a132f-ba27-43a6-a400-13d19da6f790
time=2026-07-16T23:11:43.132+09:00 level=WARN msg="Audit logs are disabled; workspace agent has not created log proxy socket" socket=/tmp/boundary-audit.sock
time=2026-07-16T23:11:43.132+09:00 level=INFO msg="Using upstream proxy" upstream_proxy=http://proxy:3080
time=2026-07-16T23:11:43.132+09:00 level=INFO msg="Start namespace-jail manager"
time=2026-07-16T23:11:43.143+09:00 level=INFO msg="Starting HTTP proxy with TLS termination" port=8080
time=2026-07-16T23:11:43.287+09:00 level=WARN msg=DENY method=GET url=https://example.com/ host=example.com
time=2026-07-16T23:11:43.288+09:00 level=INFO msg="Command completed, shutting down..."
time=2026-07-16T23:11:43.288+09:00 level=INFO msg="Stop namespace-jail manager"

• unit test result
• e2e test result