protondrive: scope proxy auth cache by client headers

[tdawe1]

Support the rclone Proton Drive backend update by fixing the proxy auth cache behavior used during SDK-era client identification.

This change is intentionally narrow. It does three things:

• scope the built-in proxy auth cache by username, x-pm-appversion, and x-pm-drive-sdk-version instead of username alone
• preserve raw username keys for custom AuthCacher implementations so external caches keep working as before
• update the live proxy tests to use a valid external Drive client identity now that the old web-account@... identity no longer works on /auth/v4/info

Without this, one client identity can reuse cached auth information created for another, which makes the proxy behavior inaccurate and can hide real upstream auth problems.

This PR is split from the underlying SDK plumbing and from the broader route/retry hardening work so the cache fix can be reviewed on its own merits.

Testing:

go test ./server

GO_PROTON_API_TEST_USERNAME=... \
GO_PROTON_API_TEST_PASSWORD=... \
go test ./server -run '^(TestServer_RealProxy|TestServer_RealProxy_Cache)$' -count=1 -v

This PR should be reviewed on top of the companion go-proton-api SDK base compatibility PR.

Companion base PR: #5