Implement AgentEndpoint client certs for upstream

[Alice-Lilith]

Adds support to AgentEndpoint for specifying a list of client certificates to use with the upstream and adds translation support from Gateway API Gateway.Spec.BackendTLS.ClientCertificateRef.

[codecov]

Codecov Report

Attention: Patch coverage is 41.47287% with 151 lines in your changes missing coverage. Please review.

Project coverage is 29.21%. Comparing base (5313c66) to head (69e5511).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rnal/controller/agent/agent_endpoint_controller.go	0.00%	85 Missing ⚠️
internal/controller/gateway/gateway_controller.go	0.00%	30 Missing ⚠️
api/ngrok/v1alpha1/zz_generated.deepcopy.go	0.00%	17 Missing ⚠️
pkg/tunneldriver/driver.go	25.00%	9 Missing ⚠️
internal/ir/ir.go	0.00%	6 Missing ⚠️
pkg/managerdriver/controller-handler.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #594      +/-   ##
==========================================
+ Coverage   29.01%   29.21%   +0.19%     
==========================================
  Files          92       92              
  Lines       14358    14551     +193     
==========================================
+ Hits         4166     4251      +85     
- Misses       9949    10053     +104     
- Partials      243      247       +4     

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