[#308] Add tezos RPC's new bigmap endpoint to morley-client

Description

Problem: At the moment, morley-client supports retrieving the value for a big_map key through this endpoint:

POST ../<block_id>/context/contracts/<contract_id>/big_map_get

However, this endpoint has limitations, and has been deprecated:

In babylonnet this entrypoint finds big_map with relevant key type and seeks for key in it; if there are multiple big_maps with the same key type, only one of them is considered (which one - it seems better not to rely on this info).

Solution: Add support for the new RPC endpoint which supersedes the one above.

GET ../<block_id>/context/big_maps/<big_map_id>/<script_expr>

Related issue(s)

Resolves #308 (closed)

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