Last and first built-in synthetic fields on list fields

**Is your feature request related to a problem? Please describe.**

Given a subgraph with a query field `entities` of type `[Entity!]!`, then `entities.first` and `entities.last` should produce a field path that represents the first and last of those entities ordered by `timestamp` (by default). If `timestamp` is not a valid field of entity `Entity`, then the field by which to order the entities can be specified using the `orderBy` argument.

**Describe the solution you'd like**
```py
sg = Subgrounds()
uniswapV2 = sg.load_subgraph("https://api.thegraph.com/subgraphs/name/uniswap/uniswap-v2")

# These two are equivalent
last_swap_id = uniswapV2.Query.swaps.last().id

last_swap_id = uniswapV2.Query.swaps(
orderBy=Swap.timestamp,
orderDirection='desc',
first=1
).id

# These two are also equivalent
last_pair_id = unsiwapV2.Query.pairs.last(orderBy='createdAtTimestamp').id

last_pair_id = uniswapV2.Query.pairs(
orderBy=Pair.createdAtTimestamp,
orderDirection='desc',
first=1
).id
```

*Originally from: cvauclair*

Look at the subgrounds codebase for where list fields and synthetic fields are defined. The issue suggests adding 'first' and 'last' methods to list fields that generate appropriate GraphQL queries with ordering. Start by examining the FieldPath class and how orderBy and orderDirection arguments are currently handled. Check existing tests for similar functionality to understand the pattern. The goal is to implement these methods to produce equivalent queries as shown in the examples.

索引モデルが issue の本文から書いたものです。