rpcdaemon: refactoring ContextTestBase as infra test utility

[canepat]

This PR splits rpc::test:: ContextTestBase into two different test utility abstractions:

• silkworm::test_util:: ContextTestBase in infra module
• silkworm::rpc::test_util::ServiceContextTestBase in rpc module

in order to remove any dependency of rpc/storage package from rpc/test_util, as a preparation step to then move storage package in db module.

After this PR, the only remaining rpc/storage dependencies in rpc are from ether::kv::Transaction and rpc::test::MockTransaction, which will be moved together with storage.

Extras

• move kEarliestBlockNumber definition from rpc to core to break dependency of rpc/storage package from rpc/core
• rename some namespace test as test_util for naming consistency