Support issue creation in separate project

Summary

Supports issue creation in a separate project by exposing ISSUE_TRACKER_PROJECT_ID environment variable. If set, issues will be created in the specified project. This enables support for custom issue trackers.

Closes #3 (closed)

Testing

Without a custom issue tracker set

#!/bin/bash

export ISSUE_BOT_API_TOKEN=$(cat my-api-token.txt)

export CI_PROJECT_ID=28421015
export CI_PIPELINE_ID=346644958
export CI_JOB_URL="https://gitlab.com/mnielsen/issue-bot-examples/-/jobs/1470620808"
export CI_PIPELINE_URL="https://gitlab.com/mnielsen/issue-bot-examples/-/pipelines/346644958"
export CI_COMMIT_SHORT_SHA=1a0f74ec
export CI_COMMIT_REF_NAME=master

# export ISSUE_TRACKER_PROJECT_ID=29096385

go run main.go

2021/08/24 11:50:59 ISSUE_TRACKER_PROJECT_ID not set, using CI_PROJECT_ID for new issues
2021/08/24 11:51:01 issue does not exist yet. Creating...
2021/08/24 11:51:01 issue created: https://gitlab.com/mnielsen/issue-bot-examples/-/issues/21

With a custom issue tracker set

#!/bin/bash

export ISSUE_BOT_API_TOKEN=$(cat my-api-token.txt)

export CI_PROJECT_ID=28421015
export CI_PIPELINE_ID=346644958
export CI_JOB_URL="https://gitlab.com/mnielsen/issue-bot-examples/-/jobs/1470620808"
export CI_PIPELINE_URL="https://gitlab.com/mnielsen/issue-bot-examples/-/pipelines/346644958"
export CI_COMMIT_SHORT_SHA=1a0f74ec
export CI_COMMIT_REF_NAME=master

export ISSUE_TRACKER_PROJECT_ID=29096385

go run main.go

2021/08/24 11:48:07 setting issue project ID to 29096385
2021/08/24 11:48:07 setting issue title prefix to issue-bot-examples
2021/08/24 11:48:08 issue does not exist yet. Creating...
2021/08/24 11:48:08 issue created: https://gitlab.com/mnielsen/my-other-project/-/issues/6

To validate:

• Check the example issue created: https://gitlab.com/mnielsen/my-other-project/-/issues/6
• Confirm that the source project's name is in the issue prefix
• Confirm that the links direct back to the source project