[storagext] Add subscribe flag instead of always waiting for success

[jmg-duarte]

Due to paritytech/subxt#1668 we can't blindly trust the following code

polka-storage/cli/polka-storage/storagext/src/runtime/client.rs

Lines 96 to 97 in 29aaad9

	// finalized != successful
	let xt_events = finalized_xt.wait_for_success().await?;

Adding a subscribe flag should solve this, at the CLI level, we can also offer that.
Something like

storagext-cli --subscribe market add-balance ... # and ignore the subscribe on commands that dont use it
# or
storagext-cli market add-balance --subscribe # which is a bit more of a pain to implement but the cli becomes cleaner\

Use one of:

• https://docs.rs/subxt/latest/subxt/blocks/struct.BlocksClient.html#method.subscribe_best
• https://docs.rs/subxt/latest/subxt/blocks/struct.BlocksClient.html#method.subscribe_finalized
  And check the blocks for the extrinsic hash

As for results, since the "non subscribe" means that you don't have a result, maybe Either will help instead of re-inventing it (Result would work but it has the Success/Error intent)