bug: fix scheduler parameters to support bools and numbers

[andyroyle]

Checklist

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• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have checked for similar issues and haven't found anything relevant.
• [x] This is not a security issue (which should be reported here: https://circleci.com/security/)
• [x] I have read Contribution Guidelines.

Changes

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• Changed type of schedule parameters from map[string]string to map[string]interface{} to enable support for bools and numbers as well as string parameters

Rationale

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Without this, only strings are supported via the api-client.

Considerations

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map[string]interface{} will transparently marshal into json without any further changes. Changing the type of the Parameters field in the Schedule struct is, unfortunately a breaking change.