Storing credentials in the profile

**Is your feature request related to a problem? Please describe.**
I'm trying to store some sensitive variables like storage account credentials in the dbt profile, and then use them in sql using `target`. So far the only feasible solution seems to be using custom parameters like log_comment `custom_settings: {'log_comment': 'myaccountkey=='}` because others are not passed to the target

**Describe the solution you'd like**
extend the profile to allow additional credentials, for example
```
extra_params:
var1: val1
```
and pass that to the target

**Describe alternatives you've considered**
setting env variables and referring to them in a macro isn't acceptable. the value must come from a profile

The issue names no files or tests; start by tracing how profile values, including custom_settings, are parsed and exposed through target. Compare that path with the requested extra_params behavior and determine how sensitive values should reach SQL. Done means additional profile credentials are available through target without requiring environment variables, with coverage for the new behavior.

Written by the indexing model from the issue text.