Embed Nats server in gitea

[TheFox0x7]

Feature Description

Since gitea and nats are both written in go, gitea can embed nats service and have it always available regardless of environment.

Why is it beneficial?

• ability to have a single provider for a feature set regardless of environment (standalone binary, container, cluster all have it available with the same feature set)
• Simplification of queues - nats can handle them replacing need for multiple providers in the first place.
• PubSub channel is already there if needed in the future
• Various services and system might benefit from having a request/reply channel
• JetStream features can be enabled if needed providing an even wider feature set out of the box
• Common system between clustered gitea and standalone

Potential issues I've identified so far:

• Migration from current user setups
• Added memory usage is unknown
• New binary size is unknown pulling just the client (but not doing much with it - only connecting) brings less than 800kB. Server adds 5.5MB.
• Redis is also used for cache so this won't be removing a dependency (aside from queue only ones but they aren't as big)
• Nats does not have counters for future rate limiting cases (they are planned to be in alpha for 2.12 release)

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It's a very low priority idea but I figured I should write it down so there's a place for discussions on this. I've seen it mentioned in relation to preparing gitea for clusters before.

Screenshots

No response