flagext: add intstring type

[titolins]

What this PR does:

• Adds a new IntString type to be used for configuration values

Which issue(s) this PR fixes:

• As mentioned in the previous mimir PR (Alerting: add new type to support both primitive and string ints mimir#9840), we're having issues with certain configuration parameters that take long integers (e.g. 2048000 gets converted to 2.048e+06)
• Adding a type that supports both strings and integers gives us the flexibility to migrate long values and not break the existing setup

Checklist

• Tests updated
• CHANGELOG.md updated

[titolins]

We've decided to use flagext.Bytes instead of introducing a new type: grafana/mimir#9845