[query] Remove lookupOrCompileCachedFunction from Backend interface

[ehigham]

In the spirit of shrinking the Backend functionality to a core set of functions, move code cache access via execute context.

[ehigham]

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• [query] Lowering + Optimisation with implict timing context #14731 : 2 dependent PRs (#14732 , #14748 )
• [query] Lift backend state into {Service|Py4J}BackendApi #14698
• [query] Move LoweredTableReaderCoercer into ExecuteContext #14696
• [query] unify backend rpc #14693
• [query] Remove persistedIr from Backend interface #14692
• [query] Remove lookupOrCompileCachedFunction from Backend interface #14691 👈 (View in Graphite)
• [query] Remove BlockMatrix persist from Backend interface #14690
• [query] Expose references via ExecuteContext #14686
• [query] Extract Backend Methods called from Python into Py4JBackendExtensions #14767
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