The block pinning is a feature required by the RPC Spec V2.

The block pinning feature should be called for subscribers of the chainHead_unstable_follow method.

Each block produced by the chainHead_unstable_follow (new block, best block, finalized block) subscription should
be pinned to offer data access for:

• block body (the old chain_getBlock RPC)
• block header (the old chain_getHeader RPC)
• storage
• runtime call

The substrate has pin and unpin functionality but it is only preserving the state of the block.

Extend the database and client to offer support for pin_block and unpin_block to reference count
the desired block and prevent it from pruning.