[GraphQL]: Add support for slot-based nested queries

[buffalojoec]

Motivation

Throughout the GraphQL schema, fields populated by an Address have been
given the type Account, which enabled nested queries on that address. Take,
for example, a lookup table account.

"""
A lookup table account
"""
type LookupTableAccount implements Account {
    address: Address
    data(encoding: AccountEncoding!, dataSlice: DataSlice): String
    executable: Boolean
    lamports: Lamports
    ownerProgram: Account
    space: BigInt
    rentEpoch: Epoch
    addresses: [Address]
    authority: Account
    deactivationSlot: Slot
    lastExtendedSlot: Slot
    lastExtendedSlotStartIndex: Int
}

You can see fields like ownerProgram and authority are queryable as
Account types, allowing full account queries to be performed on these nested
fields.

We can enable the same types of nested queries for block on any fields that are
populated with Slot.

Details

This is actually quite trivial to implement, given the relationship of type definitions
and resolvers defined in this library. Here's an example.

"""
A lookup table account
"""
type LookupTableAccount implements Account {
    address: Address
    data(encoding: AccountEncoding!, dataSlice: DataSlice): String
    executable: Boolean
    lamports: Lamports
    ownerProgram: Account
    space: BigInt
    rentEpoch: Epoch
    addresses: [Address]
    authority: Account
    deactivationSlot: Block   # `Block` type, instead of `Slot`
    lastExtendedSlot: Block   # `Block` type, instead of `Slot`
    lastExtendedSlotStartIndex: Int
}

LookupTableAccount: {
    authority: resolveAccount('authority'),
    data: resolveAccountData(),
    ownerProgram: resolveAccount('ownerProgram'),
    // Block resolvers hooked up.
    deactivationSlot: resolveBlock('deactivationSlot'),
    lastExtendedSlot: resolveBlock('lastExtendedSlot'),
},

The main question here is "should we do this?" and unless we can produce
salient arguments against it, I believe we should.