Support dynamic SQL provider for @Query methods

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Introduce a QueryProvider SPI that generates the SQL statement for a repository query method at runtime, based on the bound parameter values, following the design discussed in #542. A provider implementation is configured via the new queryProviderClass attribute of @Query, while parameter binding, query execution, and result mapping remain handled by Spring Data JDBC:

interface ProductRepository extends CrudRepository<Product, String> {

    @Query(queryProviderClass = ProductQueryProvider.class)
    List<Product> findByName(@Param("name") @Nullable String name);
}

class ProductQueryProvider implements QueryProvider {

    @Override
    public String getQuery(SqlParameterSource source) {

        String sql = "SELECT * FROM product";
        return isNotNull(source, "name") ? sql + " WHERE name = :name" : sql;
    }
}

Changes:

• New QueryProvider functional interface (getQuery(SqlParameterSource) plus isNotNull/isNotEmpty default helpers) in org.springframework.data.jdbc.repository.query.
• New @Query(queryProviderClass = …) attribute, mutually exclusive with value and name. The configured class gets instantiated through its default constructor, following the rowMapperClass conventions.
• StringBasedJdbcQuery obtains the SQL statement from the provider on every invocation after parameter binding. Custom RowMapper/ResultSetExtractor configuration applies as for any other @Query method. Value expressions and #{#tableName} preprocessing are not applied to provider-generated SQL.
• DeclaredQueryLookupStrategy resolves provider methods first and reports invalid combinations (value/name together with a provider, non-instantiable provider classes) with IllegalArgumentException so that CREATE_IF_NOT_FOUND does not silently fall back to query derivation.
• Query methods using a QueryProvider are excluded from AOT repository generation and resolve at runtime.
• Reference documentation section for dynamic queries.

Closes #542