feat: implement simple Avro schema validation utility

[hhromic]

Hi !
First of all, thank you for this library. It is really useful and well designed/implemented. Kudos!

I wanted to contribute back a small command-line utility I developed using this library that turned out very useful for us. The tool allows to validate Avro schemas, i.e. check that they are well-formed/parseable. We use this tool mostly in our CI/CD pipelines as an extra layer of protection to schema syntax mistakes/breaking.

I based the code pretty much on the existing avrogen utility code for consistency and also added unit tests.
Note that I had to manually clear the global schemas cache after each subtest in TestAvroSv_ValidatesSchema, otherwise a previous subtest state influences the next one.

Given that I was there, I also fixed an existing minor typo in the README (./app instead of avrogen).

I hope you are okay with this contribution, if you prefer not to accept it, no hard feelings!

[nrwiersma]

Thanks for the contribution 🎉 This seems like a simple but valuable addition.

[nrwiersma]

LGTM