Add ConfigProvider to make use of environment variables

So far it is not possible to inject additional configurations stored in environment variables. This topic came up in several projects and would be a useful feature to have as a Kafka config feature similar to file/directory providers, e.g.:

config.providers=env
{{}}

config.providers.env.class=org.apache.kafka.common.config.provider.EnvVarConfigProvider

ssl.key.password=${env:<...>:KEY_PASSPHRASE}{}

Link to KIP: KIP-887