fix: Update searchInvoices to use array for external ids + use lower camel case for search param

[mortondaniel]

Fixes

I noticed that the searchInvoices API actually expects the search parameter (e.g. externalId) in lowerCamelCase.
However the SDK currently sends it in UPPER_SNAKE_CASE.

Additionally, if using externalId filtering, it should be an array of values, rather than a single value (according to the API + from my testing)

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have titled the PR appropriately
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• I have added tests that prove my fix is effective or that my feature works
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• I have added inline documentation to the code I modified

[mortondaniel]

@Aman-Aalam could you please review (Github isn't letting me add reviewers) - thanks!

Also cc @tobyheighway @sa-canva