Feat: optimize performance on hot paths and reduce allocations

[VoidClancy]

1-runtime:
- Avoid Interface dispatch on hot paths, use a struct embedded in the Queries struct, and flattened-out props
- Replace Dialect interface with a struct, holding dialect sepecific functions and keywords for an easily-extensible workflow
- Define WriteQuotedIdent(), WritePlaceholder() reciever methods on Dialect, zero alloc, writes directly in the string builder
- Define Quote, BindVar(), FormatLimitOffset() methods on Dialect
- define BuildConflictClause() as Dialect method using field-based conflict keywords

2-client:
- define a mutex on the Queries struct, stmtCache, to prepare and cache sql queries, dialect sepcific quoteChar, and placeholderFmt to prevent interface dispatch on hot paths (should be made into a dialect struct embedded in the Queries struct for future extensibility)
- define Prepare() with Rlocks/unlocks, use it in query() and queryRow()
- remove postgresDialect/sqliteDialect types and old Queries.dialect field

3- builders_create:
-remove generic executeInsert, executeCreateMany/AndReturn, remove groupRowMapsByKeys() and partitionRowMaps(), that takes a []map[string]any as it is model-specific now

4- builders_query:
- remove generic executeFindOne/FindMany, executeSingle/ManyWithRelations

5- model_create:
- inline executeCreate, executeCreateMany/AndReturn, early return direct d.runCreate path when no extensions, closure onion when extensions present
- define runCreate, runCreateFallback, runCreateMany, runCreateManyAndReturn (model-specific SQL execution)
- define buildBulkInsertSQL using run-time colMask union (no generic []map[string]any)
- define partitionXXInputs using colMask (dialect-aware, no generics)
- remove pkCols inline slice literal, use package-level {{ lowercase .Model.Name }}PKCols

6- model_query:
- add early return check on executeFindUnique/FindFirst/FindMany for if no extensions were present in the exec functions, it executes the underlying exec function, or define a closure, handles extensions (onion pattern), then execute the underlying exec function if there were any extensions present
- define shared queryOne() and queryMany() for single and bulk ops without duplication

7- relations_runtime.gotpl:
- remove generic buildBulkInsertSQL, mapToColsVals
- buildSingleInsertSQL uses Dialect.WriteQuotedIdent/WritePlaceholder/BuildConflictClause

8-model_structs:
- define colMask() that returns a bit mask of columns that are set, prevents extra allocation for no reason
- move PKCols to be a package level var instead of re-defining on each query for bulk ops, saves some alloc

9- generator/helpers:
- fix isKnownDefaultFunc to skip empty-string entries