add generate_quantities timing support

[avehtari]

Closes #1070 after the next CmdStan release.

CmdStan develop version adds now timing information to standalone generated quantities csv. This PR adds CmdStanR support so that the timing reporting with $generate_quantities() matches $sample().

For CmdStan 2.38 and earlier the timing reported stays the same, that is, showing 0.0 seconds.

Example output with CmdStan 2.39+

Running standalone generated quantities after 4 MCMC chains, all chains in parallel ...

Chain 3 finished in 1.3 seconds.
Chain 4 finished in 1.4 seconds.
Chain 2 finished in 1.7 seconds.
Chain 1 finished in 1.9 seconds.

All 4 chains finished successfully.
Mean chain execution time: 1.6 seconds.
Total execution time: 2.0 seconds.

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