blockservice: move to single unique struct and add WithContentBlocker option

The idea is to have a THE blockservice object, this means we wont ever have an issue where multiple competitive features play poorly with each other due to how nested blockservices could do.

Let's say we add multi-exchange based on content routing, do we want to have to handle the mess this could create with nested blockservices ?

It implements features for ipfs-shipyard/nopfs#34.

Author: Jorropo

CHANGELOG.md

@@ -18,6 +18,7 @@ The following emojis are used to highlight certain changes:
 
 - `blockservice` now has `ContextWithSession` and `EmbedSessionInContext` functions, which allows to embed a session in a context. Future calls to `BlockGetter.GetBlock`, `BlockGetter.GetBlocks` and `NewSession` will use the session in the context.
 - `blockservice.NewWritethrough` deprecated function has been removed, instead you can do `blockservice.New(..., ..., WriteThrough())` like previously.
+- `blockservice` now has `WithContentBlocker` option which allows to filter Add and Get requests by CID.
 
 ### Changed