feat: support custom transaction extension values

[DenzelPenzel]

Summary

Allow callers to provide metadata-aware values for custom transaction extensions without implementing a new Config:

let params = PolkadotExtrinsicParamsBuilder::new()
    .custom_extension("RestrictOrigins", true)
    .build();

The value is encoded against the extension type from runtime metadata. This fixes signing on chains such as Paseo Asset Hub, where RestrictOrigins contains a bare bool.

Fixes #2265.

Implementation

• Preserve known default extension behavior and parameter access.
• Encode custom values using runtime metadata.
• Reject duplicate names, overrides of known extensions, and non-empty implicit data.
• Restore the output buffer on encoding errors.

Breaking changes

DefaultTransactionExtensions<T> and its Params type changed from public tuple aliases to structs with private fields, so that custom extension values can be carried alongside the known ones. The tuples live on under new names: KnownDefaultTransactionExtensions<T> and KnownDefaultExtrinsicParams<T>.

Code which configures transactions via the builder and passes the params through opaquely is source-compatible and produces byte-identical transactions. Code which relied on the tuple shape of the params migrates as follows:

Before	After
constructing the params tuple manually	DefaultExtrinsicParams::from_known(tuple)
mutating an element, e.g. params.5 = ...	params.known_mut().5 = ...
reading an element, e.g. &params.7	&params.known().7

Given the above, this should land in 0.51 rather than a 0.50.x patch.

Notes

• The V5 extension-version selection part of the issue is deliberately out of scope here; it is tracked in Always use transaction extensions version 0 when working with v4 transactions, and find latest match when using v5 #1998.
• Custom authorization extensions and custom extensions with non-empty implicit data are not supported; the latter returns a clear error naming the extension.
• Includes a drive-by cargo fmt fix for two test asserts that landed unformatted on master.