add option to disable Jackson annotations

[tangiel]

Endpoints previously bundled the Jackson annotation introspector,
which didn't really have an effect when Jackson was a vendored and
repackaged dependency. Now that it's an explicit dependency, the
annotations can clash with previous Endpoints behavior when the
annotations are used outside of an Endpoints context.

This change adds an option to disable the Jackson behavior, which is
done so by setting ENDPOINTS_JSON_DISABLE_JACKSON_ANNOTATIONS to a
truthy value or the endpoints.json.disableJacksonAnnotations system
property.

Consequently, this change also refactors MapSchemaFlag into a general
EndpointsFlag class for future use, as it is more flexible and easy to
implement than the ServletInitializationParameters flags.

[codecov-io]

Codecov Report

Merging #169 into master will increase coverage by 0.02%.
The diff coverage is 90.47%.

@@             Coverage Diff              @@
##             master     #169      +/-   ##
============================================
+ Coverage     80.22%   80.24%   +0.02%     
- Complexity     1712     1715       +3     
============================================
  Files           158      158              
  Lines          5698     5704       +6     
  Branches        751      755       +4     
============================================
+ Hits           4571     4577       +6     
- Misses          843      844       +1     
+ Partials        284      283       -1

Impacted Files	Coverage Δ	Complexity Δ
.../api/server/spi/config/model/SchemaRepository.java	92.42% <100%> (ø)	38 <0> (ø)	⬇️
...va/com/google/api/server/spi/ObjectMapperUtil.java	51.11% <100%> (ø)	5 <0> (+1)	⬆️
...gle/api/server/spi/config/model/EndpointsFlag.java	88.23% <88.23%> (ø)	5 <5> (?)
...c/main/java/com/google/api/server/spi/EnvUtil.java	70% <0%> (ø)	8% <0%> (ø)	⬇️
.../server/spi/discovery/CommonPathPrefixBuilder.java	100% <0%> (+5.55%)	9% <0%> (+1%)	⬆️

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