[RFC] Allow multiple envvar prefixes. was: Switch env with additional env variables

I am wondering whether it exists some simple method to switch the env label with additional env variables.

I am writing a SDK for TencentCloud. I want to the environment variables in the runtime of function computing. A environment variable, `"TENCENTCLOUD_RUNTIME"="SCF" `, can be used to identify the environment. I hope to use them as additional default value or replace some addtional default value, for example, "TENCENTCLOUD_SECRETID" in replace of SDK's "QCLOUDSDK_SECRET_ID"。

I cannot understand how to use custom loader, or use proper parameters to control settings class.

In addition, the docs have no detailed description about "@str format" and "@jlina " with env. I cannot find proper ways to deal with more complicated condition, for example, "TENCENTCLOUD_SECRETID" only exists when access role is setted.