Add Bearer token support

[droopy4096]

Investigating #1884 it turned out that Bearer token is not being passed along from config file, resulting in described issues. This PR adds Bearer Token support for authentication.

This fix was tested with GitLab Kubernetes Agent Server successfully.

[droopy4096]

pipeline failure seems to be unrelated to the changes and intermittent. There's no way for me to re-run the job.

[droopy4096]

inspired by docker/cli#2540

[droopy4096]

@tonistiigi would you mind taking a look at this?

[droopy4096]

@AkihiroSuda 🏓 ☝️

[crazy-max]

@droopy4096 Needs docs update in follow-up if you can do it 🙏

• https://github.com/docker/buildx/blob/master/docs/reference/buildx_create.md#kubernetes-driver-1
• https://github.com/docker/docs/blob/main/build/drivers/kubernetes.md

[droopy4096]

@droopy4096 Needs docs update in follow-up if you can do it pray

* https://github.com/docker/buildx/blob/master/docs/reference/buildx_create.md#kubernetes-driver-1
* https://github.com/docker/docs/blob/main/build/drivers/kubernetes.md

thanks @crazy-max will see what can be added... However since this PR did not introduced any new functionality and rather fixed what was expected to work I'm not certain what should be documented.

[crazy-max]

thanks @crazy-max will see what can be added... However since this PR did not introduced any new functionality and rather fixed what was expected to work I'm not certain what should be documented.

My bad! No functional changes indeed.