Adding logic to clear terminal screen when running new plan

[Aayyush]

No description provided.

[Aayyush]

When re-initializing a plan, we want to clear the terminal UI. There are two ways to do it:

• Clear the terminal UI when "Initializing the backend..." msg is received (chosen approach)
• When atlantis plan is run, find the exact ws connection that needs to clear the terminal UI and send a CLEAR msg. Since we are a stateless service, I don't think it is possible to retrieve a particular ws connection.

This approach relies on the fact that every time a plan is run, the first log msg is Initializing the backend... It is very unlikely that this will be changed in the future imo.

[msarvar]

@Aayyush as discussed in our daily meeting you can use Clear function to send a custom message to the UI that will clear the logs.

[Aayyush]

/ptal @msarvar @nishkrishnan

[msarvar]

looks great! Let's get it merged