Non transient context objects for ThreadContext

[rjernst]

ThreadContext currently provides a means of transferring headers from an incoming request through to alternate worker threads. Some internal services could use the ability to attach arbitrary objects instead manipulating string headers.

Some things that may be able to make use of such general object attachment:

• system indices, to pass patterns allowed for system indices through a request
• licensing, to pass through a frozen license state so swapping a license mid request does not result in undefined behavior

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Core)