Add option to specify a client timeout

[maraino]

Description

This commit adds a new option to specify the client timeout to the CA client.

Fixes #2176

[hslatman]

Can you also set a default timeout? I believe without a default timeout set, it will just wait until the server disconnects based on its timeouts (if any). See the output for different cases below. I know this client will be used with our CA, and we're in control over those timeouts, but it's generally a good practice to set a timeout in any http.Client that is created.

The WithContext methods can also be used, and they seem to behave as expected. However, we're not setting timeouts through the context in places where we use those methods.

# timeout on connection triggered by server; no (default) timeout in the HTTP client or context:
$ go run cmd/step/main.go ca health 
client GET https://127.0.0.1:8443/health failed: stream error: stream ID 1; INTERNAL_ERROR; received from peer

# timeout on the context from the client side:
$ go run cmd/step/main.go ca health
client GET https://127.0.0.1:8443/health failed: context deadline exceeded
exit status 1

# timeout on the client:
$ go run cmd/step/main.go ca health
client GET https://127.0.0.1:8443/health failed: context deadline exceeded (Client.Timeout exceeded while awaiting headers)
exit status 1