[rush] Consolidate pnpm options handling into InstallHelpers.resolvePnpmSettings

Read the pnpm options in a single place that derives both the common package.json "pnpm" field and a populated PnpmWorkspaceFile, removing the duplicated pnpm-version gating between generateCommonPackageJsonAsync and WorkspaceInstallManager.prepareCommonTempAsync.

Author: iclanton