EVM Proxy-server: empty shell of collect phase of blocks proposition

Sub-task of #4861 (closed).

The phase 1 (or collect phase) consists of:

1. Collecting the EVM block propositions;
2. Fill the delayed inbox with other transactions;
3. Put the block propositions in the pending block queue

In the mockup kernel we will consider only (1.) and (3.) for now.

We will consider that:

• There is one block proposition per tezos level;
• The block proposition consists of all transactions found in the inbox;

Even though, the code needs to be generic enough so it can be replaced by the real implementation of block propositions. For instance, the pending blocks queue needs to be list of pendings blocks even though it'll be only a singleton for our needs. As we have not yet defined what's a transaction (see #4876 (closed)), the transactions can just be bytes.