feat(keyring-api): add base32 private key encoding

[ccharly]

Adding a new encoding for private keys (will be used by Stellar).

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Note

Low Risk
Additive enum/struct extension with tests only; no changes to encoding logic or existing format behavior.

Overview
Adds base32 as a supported private key string encoding in the v2 keyring API, alongside existing hexadecimal and base58.

The change extends PrivateKeyEncoding / PrivateKeyEncodingStruct in private-key.ts, so encoding: 'base32' is accepted for private-key import (ImportPrivateKeyFormat, CreateAccountOptions private-key import), export (ExportAccountOptions, ExportedAccount), and related capability format declarations. Runtime validation and TypeScript assignability are covered with new struct and test-d cases; the unreleased changelog notes exportAccount support for this encoding (intended for Stellar).

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