feat: Add support for serde behind feature flag

[strykejern]

What kind of change does this PR introduce?

Add support for serde behind feature flag for insert, upsert and update
Might fix #35

What is the current behavior?

Library only supports body where type is Into<String>

What is the new behavior?

When enabling the feature flag "serde", the body parameter for the mentioned functions should now be serde::Serialize

Additional comments

I have tried to make the serde functionality "opt-in", which should mean that by default we keep the old functionality.

[soedirgo]

Thanks! Can you make it use the json optional feature from reqwest instead? That avoids depending on serde directly.

[strykejern]

Thanks! Can you make it use the json optional feature from reqwest instead? That avoids depending on serde directly.

I can't see that reqwest exports the necessary traits and functions from serde for us to use it in these functions? Maybe I'm misunderstanding the suggestion, but I think we need to depend on serde for this to work. As I wrote above; this implementation should only depend on serde if the serde feature is enabled.

Are you also suggesting to use the .json() function in the build() function in Builder?

Am I understanding your comment correctly?