Oauthbearer SASL implementation

[fresh-borzoni]

Add SASL/OAUTHBEARER authentication support (KIP-255)

Adds OAUTHBEARER mechanism for OAuth 2.0 token-based authentication with Kafka 2.0+.

Usage:

KafkaEx.Auth.Config.new(%{
  mechanism: :oauthbearer,
  mechanism_opts: %{
    token_provider: fn -> {:ok, jwt_token} end,
    extensions: %{"traceId" => "optional"}  # KIP-342
  }
})

Changes:

lib/kafka_ex/auth/sasl/oauthbearer.ex - new mechanism module
lib/kafka_ex/auth/sasl.ex - register mechanism
Tests and docs