feat: Add EnvoyProxyTemplate for global Envoy Proxy defaults

[mgs255]

Add EnvoyProxyTemplate for global Envoy Proxy defaults

Introduce EnvoyProxyTemplate in EnvoyGateway provider configuration to define default settings for all Envoy Proxy deployments. Individual EnvoyProxy resource settings take precedence over these template defaults.

This change includes:

• Add EnvoyProxyTemplate field to EnvoyGatewayKubernetesProvider API
• Implement merge functions for combining template defaults with more specific configurations
  • Container specs (image, env vars, resources, volume mounts)
  • Pod specs (annotations, labels, volumes, tolerations, affinity)
• Extend helm chart to support configuring default Envoy Proxy image settings via new envoyProxyTemplate object.
• Add unit tests for merge logic

Fixes: https://github.com/envoyproxy/gateway/issues/4764

[codecov[bot]]

Codecov Report

:x: Patch coverage is 75.00000% with 13 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 72.35%. Comparing base (76d4076) to head (c198efe). :warning: Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/gatewayapi/contexts.go	53.84%	5 Missing and 1 partial :warning:
internal/gatewayapi/envoyproxy_merge.go	83.33%	2 Missing and 2 partials :warning:
internal/gatewayapi/runner/runner.go	0.00%	2 Missing :warning:
api/v1alpha1/envoygateway_helpers.go	80.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7698      +/-   ##
==========================================
- Coverage   72.40%   72.35%   -0.06%     
==========================================
  Files         234      235       +1     
  Lines       34562    34609      +47     
==========================================
+ Hits        25026    25041      +15     
- Misses       7748     7772      +24     
- Partials     1788     1796       +8     

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