Add ability to pass prefix

[denis-savran]

Summary

Added prefix keyword argument to environ.to_config(), environ.generate_help() and related class methods.

This allows to dynamically change a prefix or to instantiate a nested configuration class without all of its parent configuration classes.

Usage example:

...
@environ.config
class PostgresConfig:
    host: str = environ.var()
    port: int = environ.var(converter=int)


@environ.config(prefix="APP")
class GrpcWriteConfig:
    postgres: PostgresConfig = environ.group(PostgresConfig)
    ...

@environ.config(prefix="APP")
class GrpcReadConfig:
    postgres: PostgresConfig = environ.group(PostgresConfig)
    ...

>>> environ.to_config(PostgresConfig, {"APP_POSTGRES_HOST": "postgres", "APP_POSTGRES_PORT": "5432"}, prefix="APP_POSTGRES")
PostgresConfig(host='postgres', port=5432)

Pull Request Check List

• Added tests for changed code.
• New or changed APIs are added to our typing examples to ensure our external type hints are sound.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.md by hand.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• Documentation in .md files are written using semantic newlines.
• Changes (and possible deprecations) are documented in the changelog.

[denis-savran]

Hello.

I decided to add a separate config for each entry point:
src/my_project/configs.py

@environ.config(prefix=ENV_PREFIX, frozen=True)
class GrpcWriteServerConfig:
    postgres: PostgresConfig = environ.group(PostgresConfig)
    logging: LoggingConfig = environ.group(LoggingConfig)
    write_dictionary_grpc = environ.group(WriteDictionaryGrpcConfig)
    write_dictionary = environ.group(WriteDictionaryConfig)


@environ.config(prefix=ENV_PREFIX, frozen=True)
class GrpcReadServerConfig:
    postgres: PostgresConfig = environ.group(PostgresConfig)
    logging: LoggingConfig = environ.group(LoggingConfig)
    read_dictionary_grpc = environ.group(ReadDictionaryGrpcConfig)
    read_dictionary = environ.group(ReadDictionaryConfig)


@environ.config(prefix=ENV_PREFIX, frozen=True)
class AlembicScriptConfig:
    postgres: PostgresConfig = environ.group(PostgresConfig)
    logging: LoggingConfig = environ.group(LoggingConfig)


@environ.config(prefix=ENV_PREFIX, frozen=True)
class CreateRowsScriptConfig:
    logging: LoggingConfig = environ.group(LoggingConfig)
    write_dictionary_grpc = environ.group(WriteDictionaryGrpcConfig)
    write_dictionary = environ.group(WriteDictionaryConfig)
    read_dictionary_grpc = environ.group(ReadDictionaryGrpcConfig)

So in my case there is no real need for this feature except for rare cases with interactive shell. Maybe you will find it useful.