Move Disk Buffering into Beta

[bidetofevil]

The disk buffering package is an important part of any usage of the OTel on mobile, and it's been used in opentelemetry-android for a while now. I think it's worth moving it from alpha to beta to encourage usage.

[LikeTheSalad]

Thanks for opening this issue, @bidetofevil

I agree it's mostly ready to go, but there are a couple of API changes that I've been planning to make based on the feedback I've collected so far, which I think should be made before moving it into a more stable status, as those changes will most likely break the current way of using the API. I've been recently busy with the OpAMP development, which is why I haven't been able to address those yet, but given that the last OpAMP PR is already created and ready for review, I was planning to move on to the disk buffering changes next. Still, I think we can move it to beta this month, though (depending on how fast the reviews are).