Build binary that imports nimbus-eth2 light client

[kdeme]

This should be seen as a first, introductory, step for starting work on Nimbus light proxy:

• Add Nimbus-eth2 repo to nimbus-eth1 vendor modules
• Create a simple binary in nimbus-eth1 that currently does not do more than just import a light client module (e.g. https://github.com/status-im/nimbus-eth2/blob/stable/beacon_chain/light_client.nim or so) and compiles.

[kdeme]

First item done by https://github.com/status-im/nimbus-eth1/commit/1e4f138574215fd796ab9f00a335991ba19153e0

[KonradStaniec]

second item done by - https://github.com/status-im/nimbus-eth1/pull/1202