feat: decompressor support

[aburan28]

What type of PR is this?

Feature

What this PR does / why we need it:

This PR adds decompressor filter support to Envoy Gateway, enabling decompression of compressed requests from clients and/or compressed responses from backends at the gateway layer. It introduces a new decompressor field on the BackendTrafficPolicy CRD, mirroring the existing compressor design pattern.

The implementation:

• Defines a new Decompression API type (api/v1alpha1/decompression_types.go) supporting three compression algorithms: Gzip, Brotli, and Zstd
• Uses a discriminated union pattern with CEL validations to ensure only the config block matching the selected type is set (e.g., if type is Gzip, brotli and zstd fields must not be present)
• Adds the decompressor field to BackendTrafficPolicySpec as a merge-patched array
• Registers envoy.filters.http.decompressor as a new EnvoyFilter type with proper filter ordering
• Translates the API types through the Gateway API layer into IR and then into xDS configuration for Envoy's decompressor filter
• Includes unit tests for the buildDecompression translation function and conformance test data fixtures
• Regenerates CRDs and deepcopy functions

Which issue(s) this PR fixes:

Fixes #7335

Release Notes: Yes

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 canceled.

Name	Link
🔨 Latest commit	d967846
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/6999f3342fe605000870b4fc

[codecov]

Codecov Report

❌ Patch coverage is 92.23301% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.67%. Comparing base (7031394) to head (d967846).

Files with missing lines	Patch %	Lines
internal/xds/translator/decompressor.go	90.90%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #8273    +/-   ##
========================================
  Coverage   73.66%   73.67%            
========================================
  Files         242      243     +1     
  Lines       37007    37108   +101     
========================================
+ Hits        27261    27339    +78     
- Misses       7826     7840    +14     
- Partials     1920     1929     +9     

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[nareddyt]

Why are these removed?