Ability to handle default bus

[svenlito]

Is your request related to a problem? Please describe.

As of now, this module expects a bus_name value to be set otherwise it'll attempt to create a default bus that is unwanted as it usually already exists. Additionally, rules creation will fail as a bus_name is required.

Describe the solution you'd like.

• in case no bus_name is defined we should use default and rather than using aws_cloudwatch_event_bus.this[0].name
• extend README to point out how to deal with a default bus.

[github-actions]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.