Add support for mTLS

[ryanfowler]

No description provided.

[Copilot]

Pull request overview

This PR adds support for mutual TLS (mTLS) authentication by introducing --cert and --key flags that allow users to specify client certificates and private keys for secure connections.

Changes:

• Added new configuration fields for client certificates and keys, with support for both separate and combined PEM files
• Updated HTTP client to use provided client certificates for mTLS connections
• Added comprehensive integration tests covering various mTLS scenarios including success cases, error handling, and file validation

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
main.go	Parses client certificate configuration and passes it to the HTTP request
internal/fetch/fetch.go	Adds ClientCert field to Request struct and passes it to the HTTP client
internal/config/config.go	Implements certificate/key parsing, validation, and merging logic with custom error types
internal/complete/complete.go	Adds cert and key flags to file path completion
internal/client/client.go	Configures TLS with client certificates for mTLS authentication
internal/cli/app.go	Defines cert and key CLI flags with validation and requirement constraints
integration/integration_test.go	Adds comprehensive mTLS integration tests with certificate generation and server setup
docs/CONFIG.md	Documents mTLS configuration options with examples

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