Migrate the code to ballerina 2.0.0-Preview1

[miyurud]

Purpose

Migrate the the Slack connector code to Ballerina 2.0.0-Preview1.

Goals

Code migration.

Approach

Fixes were mainly introduced to init() and definition of Errors.

User stories

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Release note

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Documentation

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Training

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Certification

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Marketing

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Automation tests

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Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? no
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

Samples

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Related PRs

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Migrations (if applicable)

This was tested on a Ballerina 2.0.0-SNAPSHOT pack which was built on 16th June 2020.

Test environment

JDK 1.8, Ubuntu 18.04

Learning

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