Add self-describing enum support to OpenAPI definition

[peterbroadhurst]

One step on the journey to #111, allowing us to list the possible values for an FFEnum type (renaming LowerCasedType).

This builds on a feature added to the OpenAPI schema generator in getkin/kin-openapi#411

Using the same approach in follow-on PRs, we can:

• Add in field-level descriptions with translation keys
• Remove the need for hard-coded schemas for complex objects like Message

[peterbroadhurst]

DCO stuff ate my code... will try and re-instate and re-open this for review 😢

[codecov-commenter]

Codecov Report

Merging #183 (c3541e3) into main (6213b08) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #183   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          199       199           
  Lines        10859     10868    +9     
=========================================
+ Hits         10859     10868    +9     

Impacted Files	Coverage Δ
pkg/fftypes/datatype.go	100.00% <ø> (ø)
pkg/fftypes/event.go	100.00% <ø> (ø)
pkg/fftypes/message.go	100.00% <ø> (ø)
pkg/fftypes/namespace.go	100.00% <ø> (ø)
pkg/fftypes/operation.go	100.00% <ø> (ø)
pkg/fftypes/transaction.go	100.00% <ø> (ø)
internal/events/webhooks/webhooks.go	100.00% <100.00%> (ø)
internal/oapispec/openapi3.go	100.00% <100.00%> (ø)
internal/tokens/https/https.go	100.00% <100.00%> (ø)
pkg/fftypes/enum.go	100.00% <100.00%> (ø)

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[peterbroadhurst]

ok - think I got my code back, and made DCO happy with my comments.

[nguyer]

Looks good to me!