Need rate limiting option for timestamp operations

[PhonicUK]

Our timestamp provider for our certificate rate limits the number of requests, so after a few files (we're doing ~40 at a shot) are signed a few will fail with HTTP 429 and the rest succeed.

An option to rate limit or insert a delay between each signing request that uses timestamps would alleviate this issue.

[PhonicUK]

Setting MDOP a lower value addressed this for us. This seems like something that will affect higher core count systems worse.

Having an independent rate limit for the timestamping server requests v.s. the actual signing logic would be beneficial, so that work can be done more in parallel.