[Request]: Improve builder performance when build context is large

### Feature or enhancement request details

Current builder archives entire build context and send it to builder VM while it also supports on-demand file provision through proxy. This was a technical decision to avoid overheads due to the large amount of gRPC calls (for file transfer) when the build context is large. However, it becomes another bottleneck, as archiving consumes lots of CPU cycles when the build context is huge.

With the recent support of docker ignore, users can selectively ignore files so that buildkit daemon reads only those files. Thus, we can still lower the number of file transfers without archiving entire build context.

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