Alerting: add new type to support both primitive and string ints

[titolins]

Context

• Helm doesn't handle big ints well - we should support using strings
• Some tenants are already hitting the new config limits we've added but now we can't increase them using configmaps because helm will convert them to scientific notation
• For flags this is not an issue
• I've merged this PR earlier, but it caused AM pods to crash since the go-yaml doesn't handle the string conversion automatically: https://github.com/grafana/deployment_tools/pull/189045
• IntString should allow us to use both a string or an integer
• Use that for config/state size limits
• Tested the flag parsing is working by running locally (we don't have a unit test for that setup and doing so requires a bit tinkering)
  

[bboreham]

Consider putting the new type here: https://github.com/grafana/dskit/tree/main/flagext

[titolins]

Consider putting the new type here: https://github.com/grafana/dskit/tree/main/flagext

Ah, that's a lot better. Tks 🙏

Converting this to a draft then, will adjust once the new type is added to dskit 👍

[titolins]

superseded by #9845