Update typing `CyHttpMessages.BaseMessage.headers`

[andercs]

What would you like?

This should be fairly straightforward, but the headers dictionary field in CyHttpMessages.BaseMessage within packages/net-stubbing/lib/external-types.ts can be expanded to include string[] as a valid value type (in addition to just string).

An example of when this might occur is when the server returns a set-cookie header that includes multiple cookies. This results in a value of string[] where each cookie is its own string.

Why is this needed?

This improves typing in IDEs/Editors and also prevents TS2339 wherein calling .filter on headers is flagged as problematic when it is not.

[jennifer-shehane]

We're open to review PRs that improve the types. https://github.com/cypress-io/cypress/blob/develop/packages/net-stubbing/lib/external-types.ts#L72:L72