FM: Message selection logic

[adlrocha]

AFAIK, Fendermint doesn't currently have any message selection logic. It checks that the transactions included in a block doesn't overflow its size limit, but doesn't follow any strategy to determine what messages to include if there are more than the ones that fit a block.

We should include a really simple strategy initially with clear abstraction to easily change them (or even allow users to choose the one that fit them best). For the initial implementation, as discussed sync we will:

• Order eligible messages in the pool by their premium (i.e. miner tip).
• Include them until the total size of the block is reached.
• (optionally) Check if there are any messages left that could fill the space left in the block after including all of the messages (if the last messages was index n, we would iterate from n to the total size s to see if there's any additional message that could fit the space left in the block).

[aakoshh]

A related issues is #349