Initial support for Commit-Boost, adding for now only the PBS module
(equivalent to MEV-Boost).

Note that this depends on [this
PR](https://github.com/Commit-Boost/commit-boost-client/pull/138) being
merged and released.

There is sometimes some ambiguity between mev-boost (go client) and
mev-boost (protocol via the Builder API), so to disambiguate within
commit boost we call the PBS module the sidecar implementing the
mev-boost protocol.
Here I followed the existing convention as much as possible even if it
sounds somewhat repetitive at times (commit-boost-mev-boost).

EDIT: the blocking PR is now merged and released in `0.3.0`

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Co-authored-by: Barnabas Busa <busa.barnabas@gmail.com>
Co-authored-by: Barnabas Busa <barnabas.busa@ethereum.org>