Add a chapter about SQL engine with knitr

[cderv]

• Showing how it works
• Documenting sql.show_interpolated (https://community.rstudio.com/t/get-sql-code-from-sql-chunk-with-variables-evaluated/136982/11 and Feature request: get evaluated code from SQL chunks (and others)  yihui/knitr#2131 from Add expand Option to SQL Engine yihui/knitr#1357), R expressions in SQL code chunks are not interpolated in the rendered output rmarkdown#2505
• Demoing glue_sql engine from glue 📦 (https://glue.tidyverse.org/articles/engines.html)
• New feature / option from added options immedate & replace to eng_SQL yihui/knitr#2128