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GH-39837: [Go][Flight] Allow cloning existing cookies in middleware (#39838)



### Rationale for this change
This is needed for https://github.com/apache/arrow-adbc/issues/1194 to facilitate better connection handling for flight clients in ADBC by copying the existing cookies over when creating a sub-client.

### What changes are included in this PR?
Creating a `Clone` method on the `CookieMiddleware` so that a user can create and hold a reference to a specific cookie middleware instance and then create new ones on the fly that copy over the existing cookies at that moment.

### Are these changes tested?
Yes.

### Are there any user-facing changes?
No

* Closes: #39837

Authored-by: Matt Topol <zotthewizard@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>