Add API endpoint for validating Actor input

[fnesveda]

We have an endpoint /acts/ACTOR-ID/validate-input which is not implemented in the client, we should implement it.

It takes the input to validate as POST payload, and optionally a build query parameter to specify the build tag against which to validate.

It returns a response with:

• HTTP status 200 and body { "valid": true }
• HTTP status 400 and body with the validation error

We should first add it to the documentation, so that we can refer to it in the docstrings. apify/apify-docs#722