exporter: allow session exporter to access refs

[tonistiigi]

based on #5907

Expose the existing Gateway API file access function to the
session exporter callback so it can make better decisions.

[tonistiigi]

@profnandaa The test failed on windows

    client_test.go:2365: readdir: entries:{path:"License.txt"  mode:493  size:5647  modTime:1741256959331475600}  entries:{path:"ProgramData"  mode:2147484141  modTime:1620456191021041900}  entries:{path:"System Volume Information"  mode:2147484141  modTime:1744353880328676500}  entries:{path:"Users"  mode:2147484013  modTime:1741257047315557900}  entries:{path:"WcSandboxState"  mode:2147484141  modTime:1744353880328676500}  entries:{path:"Windows"  mode:2147484141  modTime:1741257054706187200}  entries:{path:"bar"  mode:493  size:9  modTime:1744353893740354600}  entries:{path:"foo"  mode:493  size:8  modTime:1744353888451985500}
    client_test.go:2396: 
        	Error Trace:	D:/a/buildkit/buildkit/client/client_test.go:2396
        	            				D:/a/buildkit/buildkit/util/testutil/integration/run.go:103
        	            				D:/a/buildkit/buildkit/util/testutil/integration/run.go:245
        	Error:      	Received unexpected error:
        	            	failed to open writer: ref buildkit/1/n1ataygfvdpsnxi8yygvwb9u1 locked for 136.7176ms (since 2025-04-11 06:44:54.4128669 +0000 UTC m=+21.478406301): unavailable

Not sure why though. It didn't look like the actual export path was failing, but the Build() function itself. I disabled the WCOW test for now.

[profnandaa]

ACK, will take a look.

[crazy-max]

Small nit otherwise LGTM