Meta transactions implementation.

The meta transaction follows a layout similar to that of a regular
transaction and can leverage the same extensions implementing the
`TransactionExtension` trait. Once signed and shared by the signer, the
relayer may submit a regular transaction with the
`pallet_meta_tx::dispatch` call, passing the signed meta transaction as
an argument.

To see an example, refer to the mock setup and the
`sign_and_execute_meta_tx` test case in
`substrate/frame/meta-tx/src/tests.rs` file.

RFC: https://github.com/paritytech/polkadot-sdk/issues/4123

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Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: command-bot <>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>