Performance: do not fetch extra post data unless needed

[levb]

Presently, processing each post autolink goes back to the server to get the Channel and Team names for it, to match the scope. https://github.com/mattermost/mattermost-plugin-autolink/blob/0c286d8baa27c03834c2240c0e20fd518b4ec83d/server/autolinkplugin/plugin.go#L104-L117

1. These should be requested lazily so if no links have Scope defined, there's no perf impact.
2. Long term we should consider a new hook MessageWillBePostedEx that would contain extended metadata (channel and team names in this case) and will not require extra roundtrips. cc @lieut-data
3. Another long-term performance improvement here would be limiting the Scope to perform only exact matches on teams and channels, rather than the present contains (by keyword). Maybe we add new fields Channel and Team that are exact matches? This would allow pre-resolving all references on Config and comparing by ID, avoiding the extra per-message RPCs.

[lieut-data]

@ali-farooq0, this will be an interesting case to think through with respect to autolink performance efforts. Knowing the RPC overhead, in particular, will influence where in the pipeline we should optimize.

[mo2menelzeiny]

Hello @levb @hanzei
I would love to work on this