WASM plugins for importers and data providers

[iamsaquib8]

An idea, not planned yet. WASM strategy plugins exist behind the off-by-default wasm-plugins feature, but there is no sign anyone uses them yet. Extending the plugin interface to importers and data providers would let the community add brokers and sources without touching core code. Decide by 1.0 whether to grow this or remove it (#205). Comment if you have built or would build a plugin.

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Moved from issue #277 to keep the issue tracker for scheduled work. Upvote or comment if you would use this; ideas with demand go back on the roadmap.