OpenAPI3 support

[rrudakov]

Known issues:

• Project uses ring-swagger as dependency to validate generated swagger
  schema. Ring-swagger uses swagger2 specs so I can't write test to validate
  OpenAPI3 generated schema at the moment.
• Any string can be used for content-type value
• Callbacks in operations are not supported by current OpenAPI3 spec due to
  circular spec calls. I can declare callbacks as {string? any?} but I
  don't think it's a good idea.

[codecov-commenter]

Codecov Report

Merging #236 into master will increase coverage by 1.80%.
The diff coverage is 97.53%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   86.23%   88.04%   +1.80%     
==========================================
  Files          14       16       +2     
  Lines        1707     2032     +325     
  Branches      175      182       +7     
==========================================
+ Hits         1472     1789     +317     
- Misses         60       61       +1     
- Partials      175      182       +7     

Impacted Files	Coverage Δ
src/spec_tools/swagger/core.cljc	97.56% <ø> (ø)
src/spec_tools/openapi/core.cljc	95.00% <95.00%> (ø)
src/spec_tools/openapi/spec.cljc	99.02% <99.02%> (ø)

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[ikitommi]

Looking good. I would like the openapi3 be renamed just to openapi, like the Swagger2 is just swagger. If openapi4 would be announced and it would be backwards compatible, users woudn't have to change the keys in the specs. e.g. :openapi3/type => :openapi4/type, would still be just :openapi/type.

[rrudakov]

@ikitommi thank you for the feedback. I've renamed openapi3 to openapi.

[ikitommi]

Big thanks!!

[miikka]

Released in [metosin/spec-tools "0.10.4"]