Add SiteTypeCode and StatisticCode enums for waterdata parameters

[thodson-usgs]

Summary

• Adds SiteTypeCode and StatisticCode as str-based enums in dataretrieval/waterdata/enums.py
• Exports both from dataretrieval.waterdata public API
• Both enums extend str, so existing callers passing plain strings are unaffected — the enums are purely additive
• SiteTypeCode covers all 56 site types from get_reference_table("site-types")
• StatisticCode covers the 17 named statistic codes (excluding the 90+ numeric percentile entries)

Usage example:

from dataretrieval.waterdata import SiteTypeCode, StatisticCode, get_monitoring_locations, get_daily

df, _ = get_monitoring_locations(site_type_code=SiteTypeCode.WELL)
df, _ = get_daily(statistic_id=StatisticCode.MEAN)

Closes #98

Test plan

• python -m pytest tests/waterdata_test.py — all 26 tests pass
• Live smoke test: get_monitoring_locations(state_name="Connecticut", site_type_code=SiteTypeCode.WELL) returns only GW sites
• Verified requests serializes enum values correctly (uses native string value, not __repr__)

[thodson-usgs]

Closing as superseded.

Hardcoding SiteTypeCode / StatisticCode as static enums has been overtaken by the API-driven path on main: get_codes (#301) returns the reference tables live, and get_queryables (#333) exposes the per-collection parameter domains. A pinned enum list would now be a second source of truth that silently drifts from the service.

The PR's own module docstring points readers at get_reference_table("site-types"), which is no longer part of the public API — a good sign of how far the surface has moved since April.

If discoverable constants are still wanted for IDE autocompletion, the better shape is generating or validating them against get_codes rather than checking in a hand-maintained table. Branch fix-enums-site-type is retained.