Feat/enable sasl types

[bubunyo]

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[Copilot]

Pull request overview

Adds support for configuring kroxy to authenticate upstream Kafka connections for PLAIN clients using SCRAM-SHA-256/512, enabling a PLAIN→SCRAM bridging mode while keeping existing SCRAM pass-through behavior intact.

Changes:

• Introduces upstream.DialSCRAM and SCRAM client-side authentication flow using franz-go’s SCRAM mechanism.
• Adds proxy-side decision logic to translate PLAIN client auth into upstream SCRAM auth when upstream.sasl.mechanism is configured.
• Documents the new configuration and behavior, and adds targeted unit/integration-style tests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
upstream/scram_test.go	New in-process fake SCRAM broker test validating DialSCRAM end-to-end.
upstream/conn.go	Adds DialSCRAM and SCRAM client handshake driver (scramAuthenticate).
README.md	Documents upstream.sasl.mechanism and explains PLAIN→SCRAM translation semantics.
proxy/translate_internal_test.go	Adds unit test for PLAIN→SCRAM translation decision logic.
proxy/listener.go	Extends ServerConfig with UpstreamSASLMechanism.
proxy/conn.go	Dials upstream via DialSCRAM when translation is enabled; improves upstream logging.
dockerfiles/kroxy.yaml	Example/commented config snippet for upstream SCRAM translation.
config/config.go	Adds upstream.sasl.mechanism config struct and validates SCRAM-only values.
config/config_test.go	Adds config coverage for valid/invalid upstream.sasl.mechanism values.
cmd/kroxy/main.go	Wires config value into proxy.ServerConfig.

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