feat: add support of http agents and proxy

[louizo]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Add the possibility to provide a custom http agent on figma-js client instantiation.
This allows figma-js to be used in different environments (proxy,...)

• What is the current behavior? (You can also link to an open issue here)

The current version of figma-js can only be used with a direct internet connection.
In specific cases such as in a company environment (example: need a proxy to connect internet), figma-js does not work.

• What is the new behavior (if this is a feature change)?

Allowing a client to change http_agent, https_agent, or/and proxy values of axios configuration enables developers to make clients that work in more environments, such as (but not limited to) using http_proxy environment if present to connect via proxy.

• Other information:

Figma-js interface is kept the same (new values are optional), so it is backward compatible.

[jasonmaire]

I would love to see this PR merged! Very useful for me as well.