Add secrets management API (list/set/remove) to TinybirdClient

[sandshoes]

Summary

• The SDK's secret() helper only emits a {{ tb_secret(...) }} placeholder reference in datafiles — it can't manage the underlying secret value, so even an all-Python-SDK project needed tb secret set on the side.
• Adds a client.secrets namespace (list() / set(name, value) / remove(name)) wrapping Tinybird's /v0/variables endpoint, matching tb secret ls/set/rm.
• set() checks whether the secret exists first (GET /v0/variables/{name}) and creates (POST /v0/variables) or updates (PUT /v0/variables/{name}) accordingly — mirroring the create-or-update logic in the Forward CLI's own secret set command.
• Verified the real endpoint contract (/v0/variables, form-encoded name/value body, variables list key) against the vendored tinybird CLI package's TinyB client (tinybird/tb/client.py) rather than guessing.

Secret-value sensitivity: list() only ever returns name/created_at/updated_at (the API itself never returns values), and the error-raising path only ever surfaces the server's error body — the value passed to set() is never interpolated into an exception message. Covered by a dedicated regression test (test_set_secret_value_never_leaks_into_error_message).

Closes https://linear.app/tinybird/issue/PROTM-2265/python-sdk-add-secrets-management-api-lssetrm

Checklist

• CI is green (lint, typecheck, test) — secrets/gitleaks fails in this sandbox only on an unrelated SSL cert error fetching its pre-commit env
• pre-commit run --all-files passes locally (blocked by the same sandbox SSL/network limitation, not by this change)
• Tests were added or updated when behavior changed
• Public API / typing changes were reviewed
• Documentation was updated (README.md) with a "Secrets Management" section
• Breaking changes are clearly documented (none — purely additive)
• CHANGELOG.md was updated when user-facing behavior changed