Proto/AS: New RPCs to obtain the estimated shared pending slashed and own pending slashed amounts.

Based on !11879 (merged)

What

This MR implements two RPCs that return, according to currently available denunciations, the

• the shared estimated pending slashed amount, and
• the staker own pending slashed amount.

Why

So stakers can estimate their pending slashed amounts.

How

• Returns the estimated shared pending slashed amount (in mutez) of a given delegate. That is, its total estimated pending slashed amount.

octez-client rpc get /chains/main/blocks/head/context/delegates/<delegate_public_key_hash>/estimated_shared_pending_slashed_amount

• Returns the estimated own pending slashed amount (in mutez) of a given contract (staker).

octez-client rpc get /chains/main/blocks/head/context/contracts/<staker_public_key_hash>/estimated_own_pending_slashed_amount

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR