Modify `getChannelId` and `convertChannelNameToId` for efficient channel ID retrieval

[KojiNakamaru]

With the latest plugin ( https://github.com/jenkinsci/slack-plugin/releases/tag/715.v1cfed1b_9c63c ), the following slackUploadFile() cannot finish in my environment, which has roughly 5300 public/private channels. slackUploadFile() internally causes Rate limited by Slack, retrying in 30000ms many times.

pipeline {
    agent {
        node {
            label 'master'
        }
    }
    stages {
        stage('send') {
            steps {
                script {
                    sh 'echo hello > hello.txt'
                    slackUploadFile(channel: 'GXXXXXXXXXX', filePath: 'hello.txt', initialComment: "test")
                }
            }
        }
    }
}

If a channel is specified by its ID, getChannelId() can avoid many conversations.list calls. 894f6ead46a7308e9535b75e2da1c459fb4f09cf allows getChannelId() to return immediately if channelName matches the pattern of ID.

I also experimentally found that conversations.list with types=public_channel,private_channel requires much more API calls than conversations.list with two separate types=public_channel and types=private_channel. 48d915242234f74ac046fd866d066130f359166d splits convertChannelNameToId() into two methods, one for types=public_channel and another for types=private_channel. A new parameter limit=999 is also added to reduce API calls further.

Testing done

The code was packaged by mvn package and passed existing tests. The resulting hpi was installed on a jenkins server (version 2.460) and was confirmed to run correctly.

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue