[Voltagent] Make all default values configurable

### Is your feature request related to a problem? Please describe. when i reading the source code, i found that many default value were hardcode , which is ugly and not easy to let user to modify as wish ### Describe alternatives you've considered 1, we need to add a rule to the code of conduct to enforce everyone not use hardcode but use a configurable value 2, we could define all the default value in a single file on the root of the project, like `defaults.ts` , so the reference could be `defaults.someDomain.someKey` 3, this `defaults.ts` value could be overwrited by environment variables and commandline option ### Additional context _No response_ ### Describe the thing to improve 1, we need to add a rule to the code of conduct to enforce everyone not use hardcode but use a configurable value 2, we could define all the default value in a single file on the root of the project, like `defaults.ts` , so the reference could be `defaults.someDomain.someKey` 3, this `default