[Story] Implement Sales tax periods resource

[markheydon]

Summary

Implement the FreeAgent Sales tax periods resource in the SDK.

Why

Period listing for sales tax. Sibling of Sales tax configuration.

Scope

In scope:

• Typed models, wrappers, and service method(s) per https://dev.freeagent.com/docs/sales_tax_periods
• Tests, sample app probe, API coverage

Out of scope:

• Sales tax configuration (sibling)

Acceptance criteria

• Implemented per FreeAgent docs
• Follows implement-endpoint and sample probe standards
• Reports entity map SDK column updated

Parent

• [Feature] Implement Reports cluster resources #68 — Implement Reports cluster resources

Goal links

• G1 (strongly typed SDK)
• G2 (developer experience)

Sequencing

After or alongside Sales tax.