Allow use of specific ContextSnapshotFactory instance

[rstoyanchev]

Currently we use private, local instances that rely on ServiceLoader mechanism. However, it would be good to allow use of a specific instance.

[rstoyanchev]

ContextSnapshotFactory was only added in context-propagation 1.0.3, and we switched to using it recently with 98d39bc in main, effectively raising the baseline for our upcoming 1.3 release. So I'm moving this change to target 1.3 as well.

[rstoyanchev]

Re-considering the approach taken. Exposing setters in many places doesn't seem ideal.