Support large file uploads with Reactive REST Client

[geoand]

Fixes: #24405

[geoand]

The reason I have not added a test for this is because to actually have a test for the problem mentioned in #24402, we would need to download a 2GB file in CI, which could be problematic for a number of reasons...

[gsmet]

@geoand hey. So we wanted to backport this one to 2.7 with Rostislav as you requested but it doesn't apply cleanly and I'm not confident enough to solve the conflict myself and hope for the best. Could you have a look at it if you have time and prepare a specific backport?

Maybe wait for our first batch of 2.7.6 backport to get in the 2.7 branch as it touches files that were updated by some other PRs we already backported.
I'll let you know when it's done.

[gsmet]

@geoand btw, let's be cautious here: if we end up having to write a brand new patch, it might not be a good idea to backport it given it's not tested so let's assert and rediscuss if needed.

/cc @rsvoboda

[geoand]

I'll let you know when it's done.

Sure, no problem

[gsmet]

@geoand we have merged what needed to be merged so you can have a look now. First let's see if it's not a whole new patch that needs to be written because in this case, we might not backport.

[geoand]

I'll take a look and let you know

[geoand]

I took a look at this and to be honest, I'd rather postpone cherry picking until we really need this.

[rsvoboda]

There is JIRA for the issue considered for 2.7.z, so the backport would be welcome.

What are the troubles with the backport? Many changes happened to the ClientSendRequestHandler.java file?

[geoand]

Yes.