Allow Provider to only serve solved captchas only and configure how many captchas are sent

[forgetso]

When Providers are purely providing a captcha service and not concerned with data labelling they will only want to serve solved captchas.

• Provider must be able to configure the number of captchas sent
• Provider must be able to configure solved or both solved and unsolved captchas are sent to Dapp User. Should be configurable from prosopo.config.ts.
• If the number of captchas n is even, there should be n/2 solved and n/2 unsolved captchas returned.
• If the number of captchas n is odd, there should be (n-1)/2 + 1 solved and (n-1)/2 unsolved captchas returned.
• Unit tests

The captcha config would be used in https://github.com/prosopo-io/provider/blob/ad023f8f0570f640652601972ad227525c9a407e/src/tasks/tasks.ts#L273