Add optional timeout parameter to endpoints

[sssoleileraaa]

Description

Once #648 is done, we will want to have a way to pass the request timeout to api calls instead of setting it on the api object directly, like we do here:

securedrop-client/securedrop_client/api_jobs/sync.py

Line 39 in c65ee1f

api_client.default_request_timeout = 40

for example.

We already have an example of how to do this with download_submission, see https://github.com/freedomofpress/securedrop-sdk/blob/5af0b200f00bc931cfa78c0923e7354aeda09d0e/sdclientapi/__init__.py#L556-L558.