increase timeout for rest client to 30 seconds

[andyroyle]

We are regularly seeing timeouts when reading contexts from the API

Error: Get "https://circleci.com/api/v2/context/<context_id>": context deadline exceeded (Client.Timeout exceeded while awaiting headers)

Tracing the code through, it seems that this rest client is used for GetContext so updating here should be the solution to this specific issue.