omni-node: --dev sets manual seal and allows --chain to be set

[iulianbarbu]

Description

This PR changes a few things:

• --dev flag will not conflict with --chain anymore, but if --chain is not given will set --chain=dev.
• OmniNode --dev-block-time requires --dev too, and that's to enable the assumed development setup that's configured throughout the codebase when --dev is present alongside a manual seal based OmniNode.
• --dev-block-time is optional and it defaults to 3000ms if not set after setting --dev.
• to start OmniNode with manual seal it is enough to pass just --dev.

Closes: #6537

Integration

Relevant for node/runtime developers that use OmniNode lib, including polkadot-omni-node binary, although the recommended way is to use chopsticks.

Review Notes

• Decided to focus only on OmniNode & templates docs in relation to it, and leave the parachain-template-node as is (meaning --dev isn't usable and testing a runtime with the parachain-template-node still needs a relay chain here). I am doing this because I think we want either way to phase out parachain-template-node and adding manual seal support for it is wasted effort. We might add support though if the demand is for parachain-template-node.
• Decided to not infer the block time based on AURA config yet because there is still the option of setting a specific block time by using --dev-block-time. Also, would want first to align & merge on runtime metadata checks we added in Omni Node here: omni-node: add metadata checks for runtime/parachain compatibility #6450 before starting to infer AURA config slot duration via the same way.

• update the docs
• [~] mention about chopsticks in the context of runtime development