feat: Implement requestBuffer

[markwinter]

What this PR does / why we need it:

This implements the new requestBuffer API using the Envoy's Buffer and BufferPerRoute filters.
API PR: #5537

If the BTP has a Gateway as a target, then we will configure the Buffer filter on the Listener. If the BTP has a HTTPRoute as a target, then we will configure the BufferPerRoute filter for that particular route. The BufferPerRoute will take precedence - I've tested having both applied and the BufferPerRoute maxBytes won

Which issue(s) this PR fixes:

Fixes #3706

Release Notes: Yes

[codecov]

Codecov Report

Attention: Patch coverage is 62.28070% with 43 lines in your changes missing coverage. Please review.

Project coverage is 65.25%. Comparing base (61474e3) to head (d6add20).

Files with missing lines	Patch %	Lines
internal/xds/translator/request_buffer.go	63.63%	22 Missing and 10 partials ⚠️
internal/gatewayapi/backendtrafficpolicy.go	52.17%	8 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5661      +/-   ##
==========================================
- Coverage   65.27%   65.25%   -0.02%     
==========================================
  Files         213      214       +1     
  Lines       34104    34217     +113     
==========================================
+ Hits        22262    22330      +68     
- Misses      10502    10533      +31     
- Partials     1340     1354      +14     

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[zhaohuabing]

@markwinter Thanks so much for working on this great feature — really appreciate the effort you’re putting in!

If the BTP has a Gateway as a target, then we will configure the Buffer filter on the Listener. If the BTP has a HTTPRoute as a target, then we will configure the BufferPerRoute filter for that particular route. The BufferPerRoute will take precedence - I've tested having both applied and the BufferPerRoute maxBytes won.

This behavior is different than other types of settings in the BackendTrafficPolicy, where the settings targeting Gateway apply to all xRoutes. This inconsistent behavior of BackendTrafficPolicy could confuse users. We also can't apply a default RequestBuffer for all the xRoutes under a Gateway.

Another small thing: when the Buffer filter is applied at the Gateway level, it’s configured on all Listeners. While this makes sense (since BackendTrafficPolicy doesn’t target Listeners directly), it might be a bit unexpected from a user perspective.

I prefer keeping the same "Override" behavior as the other settings, and adding a Shared field in the RequestBuffer, similar to what we have done with the shared global ratelimit. We'll need to implement a "merge" mechanism to allow two-level request buffer settings. This is tracked at #1934 .

Shared global rate limit:

gateway/api/v1alpha1/ratelimit_types.go

Lines 55 to 62 in 61474e3

	// Shared determines whether the rate limit rules apply across all the policy targets.
	// If set to true, the rule is treated as a common bucket and is shared across all policy targets (xRoutes).
	// Default: false.
	//
	// +optional
	// +notImplementedHide
	// +kubebuilder:default=false
	Shared *bool `json:"shared,omitempty"`

cc @envoyproxy/gateway-maintainers

[arkodg]

hey @markwinter as @zhaohuabing suggested, can the implementation push all the intent to per Route, and we can enhance this in the future with a shared api field to be able to set common per gateway buffers ?

[markwinter]

@arkodg @zhaohuabing Thanks for the feedback. On holiday for a week and will return to the PR afterwards 👍