Add set request attribute #1080

[Fabio3rs]

The issue #1080 has some time and I didn't saw pull requests, I think this is very useful, so I make some code that do it.

I created two commits:

• the code
• the ninja clang-format that the git commit scripts asks

Suggestions for more tests? Or names? Thanks!

[andreastedile]

Hi, I am not a contributor of Pistache, but I can provide the following feedback.

• getData, tryGetData and getDataAs: I would personally condense these functions into one single function returning a std::optional. The optional should be parametrized with the type of data I want it to hold (just like what getDataAs currently does).
• I prefer setData instead of putData: as a user, knowing that there is a get function, I would look for a set corresponding one rather than a put one.
• I am not 100% sure that removeData should throw an error if the key does not exist.
• In my opinion, the naming of the functions can be improved to reflect their intent. What we want to do is to set and get the attributes of a request: setAttribute and getAttribute are intuitive to me, whereas I would have more trouble understanding the intent of setData and getData (what does "data" refer to? is it perhaps the HTTP body?).

[Fabio3rs]

Hi. Thank you! I think your observations are good, I was trying to make similar to the Pistache::Tcp::Peer code https://github.com/pistacheio/pistache/blob/master/include/pistache/peer.h#L54

I am not sure how to proceed is this case.