[US-1146] Using custom http.Client in digital signature timestamp service

[3ace]

This PR adds pdf_sign_custom_client.go to showcases how to use a custom timestamp client.

Other updates is to implements linter suggestion.

[Copilot]

Pull Request Overview

This PR demonstrates how to use a custom HTTP client with the timestamp service in digital signature operations and updates multiple files to use modern Go standard library functions by replacing deprecated ioutil calls with their os package equivalents.

Key Changes:

• Adds a new example pdf_sign_custom_client.go showcasing custom timestamp client configuration with HTTP timeout settings
• Replaces deprecated ioutil.ReadFile and ioutil.WriteFile with os.ReadFile and os.WriteFile across multiple signature example files
• Updates import statement in pdf_sign_external_google_cloud_kms.go to use the current kmspb package path

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
signatures/pdf_sign_custom_client.go	New example demonstrating custom timestamp client usage with configurable HTTP timeout
signatures/pdf_sign_twice_visible_annotation.go	Replaced deprecated ioutil.WriteFile with os.WriteFile
signatures/pdf_sign_timestamp.go	Replaced deprecated ioutil.ReadFile with os.ReadFile
signatures/pdf_sign_pkcs12.go	Replaced deprecated ioutil.ReadFile with os.ReadFile
signatures/pdf_sign_pem_multicert.go	Replaced deprecated ioutil.ReadFile with os.ReadFile in two locations
signatures/pdf_sign_pades_b_t.go	Replaced deprecated ioutil.ReadFile with os.ReadFile in two locations
signatures/pdf_sign_pades_b_lta.go	Replaced deprecated ioutil.ReadFile with os.ReadFile in two locations
signatures/pdf_sign_pades_b_lt.go	Replaced deprecated ioutil.ReadFile with os.ReadFile in two locations
signatures/pdf_sign_pades_b_b.go	Replaced deprecated ioutil.ReadFile with os.ReadFile in two locations
signatures/pdf_sign_external_google_cloud_kms.go	Multiple updates: replaced ioutil functions, updated kmspb import, improved defer error handling

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[anovik]

LGTM

[sampila]

looks good to me