Add "as_of" columns to oracle output target data

[bsweger]

Closes #296

Add sequence_as_of and tree_as_of columns to the oracle output target data files. We didn't talk about the latter in the call earlier this week, but if we're adding as a hedge against out-of-order data creation, I think it makes sense to have both.

This PR also fixes a few issues I noticed when making the above updates:

• get_target_data.py wasn't listed in src/README.md
• some of the input parameters in get_target_data.py had old help text

---

Edited to add some terminal output from a local test of the updated file:

In [1]: import polars as pl

In [2]: updated_oracle = pl.read_parquet("~/Downloads/target-data-2024-10-23/oracle-output/nowcast_date=2024-10-23/oracle.parquet")

In [3]: updated_oracle.columns
Out[3]:
['location',
 'target_date',
 'clade',
 'oracle_value',
 'nowcast_date',
 'sequence_as_of',
 'tree_as_of']

In [4]: updated_oracle.head()
Out[4]:
shape: (5, 7)
┌──────────┬─────────────┬───────┬──────────────┬──────────────┬───────────────┬────────────┐
│ location ┆ target_date ┆ clade ┆ oracle_value ┆ nowcast_date ┆ sequence_as_o ┆ tree_as_of │
│ ---      ┆ ---         ┆ ---   ┆ ---          ┆ ---          ┆ f             ┆ ---        │
│ str      ┆ date        ┆ str   ┆ f64          ┆ date         ┆ ---           ┆ date       │
│          ┆             ┆       ┆              ┆              ┆ date          ┆            │
╞══════════╪═════════════╪═══════╪══════════════╪══════════════╪═══════════════╪════════════╡
│ AL       ┆ 2024-09-22  ┆ 24A   ┆ 0.0          ┆ 2024-10-23   ┆ 2025-01-07    ┆ 2024-10-21 │
│ AL       ┆ 2024-09-22  ┆ 24B   ┆ 0.0          ┆ 2024-10-23   ┆ 2025-01-07    ┆ 2024-10-21 │
│ AL       ┆ 2024-09-22  ┆ 24C   ┆ 0.0          ┆ 2024-10-23   ┆ 2025-01-07    ┆ 2024-10-21 │
│ AL       ┆ 2024-09-22  ┆ 24E   ┆ 0.0          ┆ 2024-10-23   ┆ 2025-01-07    ┆ 2024-10-21 │
│ AL       ┆ 2024-09-22  ┆ 24F   ┆ 0.0          ┆ 2024-10-23   ┆ 2025-01-07    ┆ 2024-10-21 │
└──────────┴─────────────┴───────┴──────────────┴──────────────┴───────────────┴────────────┘

[bsweger]

@elray1 @nickreich Should we hold off on this PR pending further conversations about finalizing the hubverse target data columns?