types/lazy: add Peek method to SyncValue

This adds the ability to "peek" at the value of a SyncValue, so that
it's possible to observe a value without computing this.

Updates tailscale/corp#17122

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I06f88c22a1f7ffcbc7ff82946335356bb0ef4622
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