Add CBLDocument_Exists private API for testing

[jeffdgr8]

May I request to add this private C API CBLDocument_Exists for the CBLDocument.exists() C++ API? This would be helpful testing my Kotlin Multiplatform library for the Linux and Windows platforms that use the C SDK, as there are tests that use this internal Document.exists() API and Kotlin Native can only interop with C, and not C++ directly.

This would be helpful in an upcoming 3.2 release as well as future releases, assuming the Document.exists() API is still used in tests.

(I had a hack to find the c4doc by memory offset in 3.1, but this is extremely fragile and broke with 3.2.)

Past conversation on this topic

[pasin]

I don't see a problem adding the function as a private API. Can you also add the function to the exported symbol file?

https://github.com/couchbase/couchbase-lite-C/blob/master/src/exports/CBL_Exports.txt

Please make sure to run the [generate_exports.sh](

https://github.com/couchbase/couchbase-lite-C/blob/master/src/exports/generate_exports.sh) script to regenerate the public symbols and add them to the commit.

(Those links are to the file in the master branch; there should be the exact same files in release/3.2 branch)