Proto: allow bakers to set a consensus key (part 2/4 : `Drain_delegate`)

This MR is the second part of MR !5106 (closed) which implements the consensus key TZIP draft. See the original MR and the corresponding Tezosagora post for more general discussion about the proposed feature.

This first MR contains the implementation of the new operation Drain_delegate and the associated command in tezos-client.

It includes !5054 (closed) and !5455 (closed), and only the following commits need to be reviewed in this MR.

Changes in proto_alpha/lib_protocol: 11 files changed, 404 insertions(+), 8 deletions(-)

Total changes: 26 files changed, 1102 insertions(+), 115 deletions(-)

• Proto: add operation Drain_delegate
• Proto/Plugin: prioritize Drain_delegate over any manager op
• Proto/Plugin: filter out Drain_delegate without enough fees
• Proto/Client: add tezos-client drain delegate <del> to <pkh>
• Proto: add a minimal tezt scenario for Drain_delegate