feat:[CI-22994]: Add Azure cache credential handling and bump BuildKit to 1.0.19

[satyakota04]

Summary

This PR adds support for self-hosted Azure Blob Storage credentials for Buildx cache import/export and updates BuildKit to a version that includes Azure cache backend support.

What Changed

Added Azure Self-Hosted Cache Credential Inputs

Introduced the following new environment variables:

• PLUGIN_HARNESS_SELF_HOSTED_AZURE_ACCOUNT_KEY
• PLUGIN_HARNESS_SELF_HOSTED_AZURE_TENANT_ID
• PLUGIN_HARNESS_SELF_HOSTED_AZURE_CLIENT_ID
• PLUGIN_HARNESS_SELF_HOSTED_AZURE_CLIENT_SECRET
• PLUGIN_HARNESS_SELF_HOSTED_AZURE_OIDC_TOKEN

These inputs are now wired into the plugin build configuration.

Added Azure Placeholder Sanitization for Cache Arguments

Implemented support for replacing Azure credential placeholders with runtime values:

• Replaces harness_placeholder_azure_creds when credentials are provided.
• Removes Azure credential attributes when values are empty.
• Applies to:
  • account_key
  • tenant_id
  • client_id
  • client_secret
  • oidc_token

Added Unit Tests

Added test coverage for:

• Azure placeholder replacement
• Azure placeholder removal when credentials are not provided

Updated BuildKit Version

• Bumped BuildKit image to harness/buildkit:1.0.18

Why

This change enables type=azure Buildx cache import/export support in self-hosted environments while ensuring credential injection remains secure and consistent with the existing S3 and GCP cache credential handling mechanisms.