Add Serialization.GzipSerializer to produce gzip compressed serialization files.

[lgoettgens]

Implements proposal 3 from #5664, with some compression detection based on the file extension.

Resolves #5664.

[antonydellavecchia]

Looks good to me.

[fieker]

This adds 2 more external dependencies. How stable are they vs how important is this feature?

[lgoettgens]

This adds 2 more external dependencies. How stable are they vs how important is this feature?

TranscodingStreams is a dependency of CodecZlib. It is just specified individually, so that we can enforce a compat bound on it.
Both packages are very lightweight, and haven't had many changes in the past few years. But that's not due to a lack of maintenance, but due to a lack of the need for any changes. They don't implement any compression themselves, but just define an interface and delegate all actual compression to the Zlib binary. Thus, I would consider them very mature, and they are the de-facto standard tool for this in the julia world. We also already use both these packages already in our testsuite.