[Enhancement]: Lazy generators for Block Tree getters

[Harrm]

Description

BlockTree has some methods that return chains as vectors. Allocating a vector for a getter is unoptimal, we can just return an iterator (external or internal) instead. C++20 coroutines and C++23 generator are a nice way to implement an external iterator. However, we should be careful about multithreaded access to block tree in this case.

Motivation

Optimization of large chain requests.

Additional context

No response

Are you planning to do it yourself in a pull request ?

Yes

[turuslan]

Maybe reuse Descent class, which preloads batches lazily

kagome/core/blockchain/indexer.hpp

Lines 14 to 17 in 71d8161

	/**
	* Cached ancestry check.
	*/
	struct Descent {