Add gradle wrapper

[vlad20012]

It's common practice to store gradle wrapper inside git repository. It make initial setup much easier for new contributors. For example, we store gradle wrapper in our repository in intellij-rust.

[cretz]

While it is common practice, I don't really agree with it and even have a README note telling you to build the wrapper yourself. In the same way you are expected to have a JDK ready when building this, so to are you asked to have Gradle ready. I don't believe this burden is too large for people building the library. Even though it's common practice, the choice is a bit subjective and I fall on the side of not committing my build tool.

[skuzmich]

Hey, I just wanted to explore the project, downloaded the code but IDE is unable to index it because newer Gradle 6.4 is incompatible with it. So now I have to manually install and manage a separate old version (Thanks for mentioning the tested version in README!) of Gradle on each and every dev environment I would want to build this project on.

I would have been a happier project explorer with a Gradle wrapper included.

[cretz]

Yeah, this is a problem with build tools changing compatibility on you...imagine if you had to download cargo with every project, or npm, or composer...gradle has chosen poorly in this case. Granted I don't really work on this project anymore, so it definitely suffers from bit rot and were I to build it again, I might consider including the dumb binary blob gradle wants.