Support PrettyJson in serialized requests when Formatting.Indented specified

[russcam]

With the move to utf8json in #3493, the internal serializer does no indentation, even when Formatting.Indented is specified.

Indentation should be performed when Formatting.Indented is specified.

[codebrain]

Will mark parameter as Obsolete or through xmldocs to indicate that pretty printing functionality is not yet supported in the new serialiser in 7.x

[russcam]

clarification: don't think we're proposing to mark as obsolete, only XML docs

[stevejgordon]

Can re-review with STJ implementation.