feat: Support multiple duplicate queryparams

[mjjs]

• Fixes Stats for multiple categories in one API call #129
• Fixes When using the client to get statistics for categories, it's not possible to submit multiple categories. sendgrid-java#702

The SendGrid API documentation states that the user can supply up to ten
categories, but there is no way to supply them using the Java library.

This commit fixes the problem by interpreting any value that has an
ampersand in it as a list of values, which will be inserted as multiple
query params.

For example, the call request.addQueryParam("categories", "cat1&cat2")
will output categories=cat1&categories=cat2 into the URL.

I also modified the testbuildUri test a bit because I would have had write
out too many permutations of the resulting URL string because the order
of the query params is not deterministic in the current implementation.

Another solution would be to store the query params in Map<String, String[]>
so we wouldn't need to split the string. Then the user could just call

request.addQueryParam("categories", "cat1");
request.addQueryParam("categories", "cat2");
// Or this if we allow variadic arguments
request.addQueryParam("categories", "cat1", "cat2");

This would, however, change the API of the library, so I didn't go for this route now.

Fixes

A short description of what this PR does.

Checklist

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• I have read the Contribution Guidelines and my PR follows them
• 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
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

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[shwetha-manvinkurke]

Could you pull in the latest changes from main?

[mjjs]

Rebased and pushed.