fix: typings for Client.setDefaultRequest

[brettwillis]

Fixes

The ClientRequest type (alias of RequestOptions) in Client.setDefaultRequest has required field url.

This means, when using Client.setDefaultRequest({ httpsAgent: new https.Agent() }), for example, TypeScript will error with field url is missing.

This PR makes the type partial, so all fields are optional which reflects the actual behaviour.

Checklist

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• [x] I have added tests that prove my fix is effective or that my feature works
• [x] ~I have added the necessary documentation about the functionality in the appropriate .md file~
• [x] ~I have added inline documentation to the code I modified~