Add ability to dynamically limit number of parallel activities

[mfateev]

Is your feature request related to a problem? Please describe.
A user wants to execute an activity only if there is enough heap space left in a process.

Describe the solution you'd like
Provide a pluggable component that the user can tailor for his needs. This component would be called with poll request and can decide to delay the poll if some conditions are not satisfied.

Additional context
https://community.temporal.io/t/workloads-metrics/4002

[Sushisource]

Closing this since the interface now exists as of #2014

Still in an experimental state, but functionality is there now and will evolve based on feedback.