Ability to "hint" config options from user-code without hard override of the user environment config

[randomir]

With the current 3 layers to provide config options (like solver or endpoint):

1. config file (all options)
2. environment variables (limited set of options)
3. keyword arguments to Client's constructor and from_config (all options)

it is impossible to "suggest" default option values (e.g. solver), but allow for user to override it on any of the 3 layers. Possible use cases:

• user wants to set a soft default in code (layer 3), but allow override from environment
• 3rd party library has a notion of preferred defaults, but wants to allow an easy override for its users

Proposal: implement a mechanism to provide config options on layer zero -- a per-Client instance default value for each config option, provided on construction.

For example, adding defaults argument to Client constructor and from_config, which would .update the Client class-level defaults for all accepted config options. Additional benefits of this are: consolidation of the defaults, clearly documenting all accepted config options in one place. It can be nicely combined with #357.