feat(serializer): Add support for ragg and svglite

[thomasp85]

I have not added a bullet to NEWS since it does not have an entry for the dev version yet and I did not want to create it in this PR (happy to do so if requested)

[CLAassistant]

All committers have signed the CLA.

[schloerke]

✅

@thomasp85 Can you please set up the dev version / news entry area? (Along with a news entry for your PR). Thank you!

[schloerke]

@thomasp85 #965 added a news entry / dev version and fixed the pkgdown error. Please add a news item and I'll merge. Thank you!

[schloerke]

Forgot during initial review. Please add an entry in

plumber/vignettes/rendering-output.Rmd

Lines 53 to 75 in b041175

	Annotation | Content Type | Description/References
	---------- | ------------ | ---------------------
	`@serializer contentType` | (user supplied) | Send response with a particular `Content-Type` header
	`@serializer html` | `text/html; charset=UTF-8` | Passes response through without any additional serialization
	`@serializer json` | `application/json` | Object processed with `jsonlite::toJSON()`
	`@serializer unboxedJSON` | `application/json` | Object processed with `jsonlite::toJSON(auto_unbox=TRUE)`
	`@serializer rds` | `application/rds` | Object processed with `base::serialize()`
	`@serializer csv` | `text/csv` | Object processed with `readr::format_csv()`
	`@serializer tsv` | `text/tab-separated-values` | Object processed with `readr::format_tsv()`
	`@serializer feather` | `application/vnd.apache.arrow.file` | Object processed with `arrow::write_feather()`
	`@serializer parquet` | `application/parquet` | Object processed with `arrow::write_parquet()`
	`@serializer yaml` | `text/x-yaml` | Object processed with `yaml::as_yaml()`
	`@serializer htmlwidget` | `text/html; charset=utf-8` | `htmlwidgets::saveWidget()`
	`@serializer text` | `text/plain` | Text output processed by `as.character()`
	`@serializer format` | `text/plain` | Text output processed by `format()`
	`@serializer print` | `text/plain` | Text output captured from `print()`
	`@serializer cat` | `text/plain` | Text output captured from `cat()`
	`@serializer jpeg` | `image/jpeg` | Images created with `jpeg()`
	`@serializer png` | `image/png` | Images created with `png()`
	`@serializer svg` | `image/svg` | Images created with `svg()`
	`@serializer bmp` | `image/bmp` | Images created with `bmp()`
	`@serializer tiff` | `image/tiff` | Images created with `tiff()`
	`@serializer pdf` | `application/pdf` | PDF File created with `pdf()`

Thank you!

---

• News Entry
• Vignette: Serializer table entry

[thomasp85]

Thanks for the review - should all be fixed now