feat: add custom capabilities & account creation types to KeyringV2

[mathieuartu]

Related to: https://consensyssoftware.atlassian.net/browse/MUL-1358

Examples

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Note

Introduces extensibility for non-standard keyrings.

• Add custom to KeyringCapabilities (api/v2/keyring-capabilities.ts) to signal non-standard createAccounts options
• Add KeyringAccountEntropyTypeOption.Custom and KeyringAccountEntropyCustomOptionsStruct; update KeyringAccountEntropyOptionsStruct switch logic (api/account-options.ts) and tests
• Add AccountCreationType.Custom and CreateAccountCustomOptionsStruct; include in CreateAccountOptionsStruct union (api/v2/create-account/*)
• Update type tests to cover new enums/options (api/v2/keyring.test-d.ts) and runtime tests (api/account-options.test.ts)
• Update CHANGELOG.md to document additions

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