rpc-v2/archive: Rename archive call method result to value (#7885)

This PR modifies the archive_call method result to align with the rpc-v2
spec:
-
https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/archive_unstable_call.md

Previously, the method result was encoded to a json containing a
"result" field, however the spec specifies a "value" field. This aims to
rectify that.

cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>