Add support for global HTTP TLS client cert

What does this MR do and why?

Some customers need to configure mutual TLS authentication for Webhooks. This commit adds support for an instance-wide client certificate via two settings in gitlab.yml:

• gitlab.http_client.tls_client_cert_file
• gitlab.http_client.tls_client_cert_password

Relates to #27450 (closed)

Changelog: added

How to set up and validate locally

You can test mutual TLS authentication via https://client.badssl.com/:

1. Run bin/rails console.
2. Run ::Gitlab::HTTP_V2.get('https://client.badssl.com'). This should fail.
3. Download the client cert: curl -o /tmp/badssl.com-client.pem https://badssl.com/certs/badssl.com-client.pem
4. In config/gitlab.yml add to the gitlab section:

    ## HTTP client settings
    # Filename of HTTP client pem
    http_client:
      tls_client_cert_file: /tmp/badssl.com-client.pem
    # Filename of HTTP client pem password
      tls_client_cert_password: badssl.com

5. Repeat step 2. This should work now.