Custom transaction extensions cannot be marked as authorization extensions

Context

DefaultTransactionExtensions lets callers supply metadata-encoded values for
chain-specific transaction extensions that Subxt has no typed support for, via
DefaultExtrinsicParamsBuilder::custom_extension.

Its is_authorization_extension implementation forwards only to the known typed
extensions:

fn is_authorization_extension(&self, name: &str) -> bool {
    frame_decode::extrinsics::TransactionExtensions::is_authorization_extension(
        &self.known,
        name,
    )
}

So a chain-specific extension supplied through custom_extension always reports
false, even if that extension authorizes the transaction.

Why it matters

In V5, frame-decode excludes the value and implicit bytes of the last
authorization extension and of every extension before it from the signer
payload. If a chain declares its own authorization extension and a user supplies
it via custom_extension, Subxt would sign over bytes the runtime excludes,
producing a signer payload the runtime will not agree with.

VerifySignature (VerifyMultiSignature) is currently the only authorization
extension Subxt knows about, so this is latent rather than an active bug, and
custom_extension documents custom authorization extensions as unsupported.

Raised in review of #2273:
https://github.com/paritytech/subxt/pull/2273#discussion_r3904192456