Covert ContextStackManager to plugin manager

Problem/Motivation

See #2354889-115: Make block context faster by removing onBlock event and replace it with loading from a ContextManager

Now that I think about it, I think ContextStackManager could be a plugin manager, with each of the context providers as plugins. Then the tagged services goes away in favour of Context Plugins and the service IDs become plugin IDs.

And
#2354889-128: Make block context faster by removing onBlock event and replace it with loading from a ContextManager

the main advantage is if say contrib provides another 20 context providers, we don't end up with another 20 services in the container, instead just the plugin manager.

Then there's the DX and consistency with other plugins. Simpler to copy a file than add a service.

Proposed resolution

Explore the option

Remaining tasks

Discuss and finalize

User interface changes

N/A

API changes

TBD

Data model changes

TBD
Follow-up to #2354889: Make block context faster by removing onBlock event and replace it with loading from a ContextManager