After wasm udf appeared, code in main, create_function_statement and schema_tables got some involvements into details of wasm engine management. Also, even prior to this, there was duplication in how function context is created by statement code and schema_tables code.
This PR generalizes function context creation and encapsulates the management in sharded<lang::manager> service. Also it removes the wasm::startup_context thing and makes wasm start/stop be "classical" (see #2737)
Closesscylladb/scylladb#19166
* github.com:scylladb/scylladb:
code: Enlighten wasm headers usage
lang: Unfriend wasm context from manager
lang, cql3, schema_tables: Don't mess with db::config
lang: Don't use db::config to create lua context
lang: Don't use db::config to create wasm context
lang: Drop manager::precompile() method
cql3, schema_tables: Generalize function creation
wasm: Replace startup_context with wasm_config
lang: Add manager::start() method
lang: Move manager to lang namespace
lang: Move wasm::manager to its .cc/.hh files
7b301f0cb9