Cache the active SDK to avoid repeated queries

[abduld]

Currently the extension tries to find the SDK path at each event. As a result you get

["INFO" - 6:50:06 PM] Found SDK with version 0.0.0
["INFO" - 6:50:06 PM] Monorepo SDK found, prioritizing that over Python environment.
["INFO" - 6:50:08 PM] Found SDK with version 0.0.0
["INFO" - 6:50:08 PM] Monorepo SDK found, prioritizing that over Python environment.
["INFO" - 6:57:21 PM] Found SDK with version 0.0.0
["INFO" - 6:57:21 PM] Monorepo SDK found, prioritizing that over Python environment.
["INFO" - 6:57:21 PM] Found SDK with version 0.0.0
["INFO" - 6:57:21 PM] Monorepo SDK found, prioritizing that over Python environment.

and it slows down every operation. This caches the SDK path if defined so it's persistent across the session

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