feat(slack): add allowlist controls for agent access

[Youcef3939]

this pull request introduces new configuration options to control which users and channels the slack plugin will process events from, adding support for allowlists and direct message restrictions. these changes enhance security and flexibility by enabling administrators to restrict access to specific users and channels, and to disable direct message handling if desired. the implementation includes environment variable parsing, enforcement in event handlers, and comprehensive tests

configuration and environment parsing:

• added allow_dms, user_allowlist, and channel_allowlist options to the SlackPluginConfig interface, with parsing functions to interpret environment variables (SLACK_ALLOW_DMS, SLACK_USER_ALLOWLIST, SLACK_CHANNEL_ALLOWLIST). [1] [2]
• updated slackPluginConfigFromEnv to set defaults and parse the new configuration options

event handling and enforcement:

• implemented the canProcessSlackEvent function to enforce allowlist and DM restrictions, and integrated it into Slack event handlers to prevent unauthorized events from being processed [1] [2] [3] [4]
• updated the plugin startup and dependency injection to pass the new configuration to event handlers

testing:

• added and updated tests to verify that allowlists and DM restrictions are correctly enforced, including cases for user/channel allowlists and DM blocking [1] [2] [3] [4]
• extended the test fixture and integration tests to support and validate the new configuration options [1] [2]

these changes provide fine-grained control over which slack events are handled, improving security and compliance for deployments

issue #146

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