add configurable timeout to rpc client

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• added environment variable FOREST_RPC_DEFAULT_TIMEOUT to configure the default RPC timeout. This is useful for testing the endpoints on a lower-end machine (like GH workers, which timeout from time to time).
• Increased the default timeout for the api compare tests to 120.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.