Alpha/RPC: add a low-level but typed RPC for context introspection
Example:
```
> tezos-client rpc get /chains/main/blocks/head/context/raw/json/\?depth\=1 | jq
{
"commitments": [],
"contracts": {
"global_counter": 0,
"index": []
},
"cycle": [
4,
3,
2,
1,
0
],
"delegates": [
"tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN",
"tz1faswCTDciRzE4oJ9jn2Vm2dvjeyA9fUzU",
"tz1ddb9NMYHZi5UzPdzTZMYQQZoMub195zgv",
"tz1b7tUupMgCNw2cCLpKTkSD1NZzB5TkP2sv",
"tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx"
],
"last_block_priority": 0,
"rolls": {
"index": [],
"next": 2000,
"owner": {}
},
"votes": {
"ballots": [],
"current_period_kind": "proposal",
"current_quorum": 8000,
"listings": [],
"proposals": []
}
}
> tezos-client rpc get /chains/main/blocks/head/context/raw/json/cycle/4\?depth\=2 | jq
{
"last_roll": [
[
0,
2000
]
],
"nonces": [],
"random_seed": "97d50852c159ada8e9f107e98f693b059ba28336c723e6cd0f6353eb3c0cb415",
"roll_snapshot": 0
}
> tezos-client rpc get /chains/main/blocks/head/context/raw/json/cycle/5\?depth\=2 | jq
{
"last_roll": [
[
1,
2000
],
[
0,
2000
]
],
"nonces": [],
"roll_snapshot": 2
}
```
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