eth-bytecode-db extractors: initialization

[rimrakhimov]

Starts blockscout extractor implementation. Initializes crate for extractors. Initialize initial migrations, according to schema.

The database migrations were intentionally implemented through the sea-orm framework to being possible to apply them to any database backend supported by sea-orm. That should allow to run the extractor locally with some local in-memory database (e.g. Sqlite)

One noticeable difference with other services design is that migrations were extracted into a root eth-bytecode-db directory instead of being implemented inside extractors crate. The reason is that in that case there would be 2 crates with the same name (migration) in one workspace: one for migrations in eth-bytecode-db and one for migrations in extractors. To solve the problem I suggest to extract migration and entity crates to root service directory.