Create new type to pair extrinsic with its source

## Issue summary

- Currently the encrypted extrinsics are individually prefixed with a source. This looks like
`externalExt := types.Extrinsic(append([]byte{byte(types.TxnExternal)}, ext...))`
This is not clean and can be confusing especially since we don't define possible prefixs anywhere.
- We should create a new type that contains the opaque extrinsic and its source as well as document what the possible sources are.

## Other information and links

- https://github.com/ChainSafe/gossamer/pull/2308#discussion_r818965438

Start by tracing the types.Extrinsic and TxnExternal references shown in the issue, then review the linked pull request discussion for context. Done means a type pairs the opaque extrinsic with its source, the possible sources are documented, and existing prefix-based uses are updated consistently.

Written by the indexing model from the issue text.