add on_exists param with behaviour fail, ignore and overwrite

[adamrtalbot]

This PR adds a feature where you can customize the behaviour when a resource exists. It has three options:

• fail (default): Fail and halt seqerakit
• ignore: Keep going and ignore the error
• overwrite: Use the existing overwrite behaviour

With this, we can repeatedly set up a 'showcase' and it will only create resources where they don't exist and launch pipelines, meaning we can be fairly frugal with resource set up.

The downside is status drift may occur, where the platform resources have diverged from the YAML templates which will not be captured and will be ignored by seqerakit. Not ideal, but a problem even for things like Terraform.

[adamrtalbot]

@ejseqera I tried to use an enum to provide some typing but it ended up feeling messier than I would like. It works well now and we can extend it to move behaviours (e.g. remove, update) but I don't like the constant str -> enum -> str behaviour.