buildkitd registry-auth-tlscontext flag not support

[wangcanfengxs]

Description

Add registry-auth-tlscontext flag to support push image to insecurity registry.

When I use harbor as my private registry. I can not push image to harbor, because

ERROR: failed to solve: failed to push harbor.xxxx.com/tenant1-project1-docker/example-backend:202404031157: failed to authorize: failed to fetch oauth token: Post "https://harbor.xxxx.com/service/token": tls: failed to verify certificate: x509: certificate signed by unknown authority

When buildkit fetch token for auth, it use https.