feat(outbound): add load balancing and circuit breaking proto surface (#556)

* fix: set build_transport in gen.rs

The tonic-prost-build configure call was missing an explicit
build_transport(true). While this is the default value, being
explicit keeps the builder chain consistent with build_client
and build_server, and prevents surprises if the default changes.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

* feat(outbound): restructure FailureAccrual for multiple accrual policies

Replace the single-field oneof wrapper with direct optional fields,
allowing consecutive-failure and success-rate policies to coexist.
The consecutive_failures field retains field number 1, so the wire
encoding is identical to the old oneof layout and existing proxies
continue to work without changes.

Add a SuccessRate nested message at field 2 with threshold, decay,
and min_requests parameters. When absent, success-rate accrual is
disabled.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

* feat(outbound): add LoadBiasConfig and load_bias fields

Introduce a LoadBiasConfig message for 429-aware load balancing.
When set on Http1, Http2, or Grpc protocol variants, the proxy
injects artificial latency penalties on rate-limited endpoints
so the P2C balancer prefers healthier alternatives.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

* feat(outbound): add RetryAfterConfig and retry_after fields

Introduce a RetryAfterConfig message for Retry-After header
handling. When set on Http1, Http2, or Grpc protocol variants,
the proxy honors Retry-After headers from 429 responses and
clamps durations to the configured maximum.

Added as field 4 on each HTTP protocol variant. The single
max_duration field caps the Retry-After value the proxy will
honor, falling back to a built-in default when absent.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

* feat(outbound): add EjectionConfig to BalanceP2c

Add pool-level ejection protection to the P2C load balancer
configuration. When set this prevents circuit breakers from
ejecting all endpoints in a load-balancing pool by enforcing
a minimum number of ready endpoints.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

* build: regenerate Rust and Go bindings

Full regeneration from updated outbound.proto after the
FailureAccrual restructure, the additions of LoadBiasConfig,
RetryAfterConfig and EjectionConfig messages, and load_bias,
retry_after and ejection fields.

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

---------

Signed-off-by: Alejandro Martinez Ruiz <amr@buoyant.io>

Author: unleashed

examples/gen.rs

@@ -20,6 +20,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     if let Err(e) = tonic_prost_build::configure()
         .build_client(true)
         .build_server(true)
+        .build_transport(true)
         .out_dir(out_dir)
         .compile_protos(&proto_files, &[proto_dir])
     {