Add support for partners

[KaloyanTanev]

Add support for partners - easier overriding of env variables and docker compose files. Implement Lido as first partner. Up until now Lido has been using lido-charon-distributed-validator-node repository, however, this creates a lot of discrepancies. Having a single repository is easier to manage and keep up to date.

N.B.: This PR is on top of #283 , that is because #283 is reworking the repository in a lot of places and it's best to be used as a ground. However, adding those changes to #283 is not suitable as it this one introduces new featureset.