Allow configuring env letter case rules for settings parser

[jasonkuhrt]

Issue derived from #275

Currently BaseSettings defaults to looking for env variables in uppercase. This issue is a feature proposal to make this behaviour configurable.

The current workaround is to use the settings config class to set up aliases, e.g.:

 class S(pydantic.BaseSettings):
      test:str

      class Config:
          fields = dict(test=dict(alias="test"))

However this is verbose and onerous to do (for the developer), if there are many such env vars.

It would be nice if the developer could do something like:

 class S(pydantic.BaseSettings):
      class Config:
          letter_case = "lower" # or:  "upper" | "any"

      test:str

About this concern raised for case insensitive option #275 (comment).

• we could log a warning or raise an error at a certain configurable threshold of
  • env var count
  • or perhaps based on measured processing time exceeding configurable budget (e.g. 1ms)
• we could cache the calculation of env2 (but probably the entire settings result would just be cached by the app)
• the env prefix could help reduce the amount of work needed to do

[samuelcolvin]

I think just a warning in the docs is enough for the size thing.