feat(secrets): named Secrets and EnvVarSecret bindings

Adds the client half of database-scoped named Secrets: a Secret is a name and
an opaque value stored by the service, and a Function binds one to the
environment variable its library already reads.

- `db.create_secret` / `alter_secret` / `list_secrets` / `describe_secret` /
  `drop_secret` on sync, async and remote connections, with the pyo3 binding
  and the Rust client behind them. There is no read API by construction rather
  than by policy: no code path returns a stored credential, and
  `describe_secret` answers with metadata only.
- `EnvVarSecret(secret=..., env_variable=...)` pairs a Secret with the variable
  it arrives in. It is a pure local constructor -- it contacts no server, so it
  cannot fail on a Secret that does not exist -- and it exists so a bare string
  in that position, which would be a credential, is a TypeError rather than a
  plausible-looking mistake that reads identically in a diff.
- `create_function(..., secrets=[...])` carries the bindings as
  `secret_bindings`, a map from variable name to Secret name. The value never
  travels: it is resolved by the service when the Function runs, which is what
  lets a rotation reach columns pinned to an older FunctionVersion.

The UDF body is unchanged and stays portable -- it reads `OPENAI_API_KEY` the
way it always did, and the binding is what puts a value there.

Squashed: the original three commits were a first design plus a rewrite of it,
so their sequence describes an interface that no longer exists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XE1UwYKsgbb3USBfkqCE6v

docs/src/python/python.md

@@ -125,6 +125,10 @@ listing a storage directory.
 
 ::: lancedb.functions.UdfDefinition
 
+::: lancedb.secrets.EnvVarSecret
+
+::: lancedb.secrets.SecretInfo
+
 ::: lancedb.functions.FunctionRegistrationRequest
 
 ::: lancedb.functions.FunctionArtifactRequest