Deprecate Callable config optoins

[soapy1]

Context

CondaStore.validate_specification and CondaStore.validate_action are settings which currently allow users to define their own functions for validating specs and actions. Additionally CondaStore.post_update_environment_build_hook allows users to specify actions to take after updating an environment As we move away from traitlets and python config files as per #1005 it does't make sense to continue to support config files that make heavy use of functionality provided by python. For example, users shouldn't be able to inject runnable code in the config file.

The following config values are Callable and should be deprecated:

• CondaStore.validate_specification
• CondaStore.validate_action
• CondaStore.post_update_environment_build_hook

Instead, it would be better to expose this type of functionality to users thru plugin hooks.

Value and/or benefit

• users shouldn't be able to inject code from the config file
  • as we move away from python config files, we will not be able to support this pattern
• users should be able to customize conda-store by using plugins