[gradle plugin] Support Gradle 4.10 (#1011)

* Support Gradle 4.10

Gradle 4.10 is bundled with Kotlin 1.60 and Kotlin DSL 1.0-rc1. The new
Kotlin DSL isn't binary compatible with the `tasks` registration used in
this plugin.

Updating to Kotlin DSL 1.0-rc1 with no other changes would require users
to update to Gradle 4.10.

As a workaround, I've modified the tasks registration being done in
OpenApiGeneratorPlugin.kt, so rather than using the Kotlin DSL's invoke,
it creates tasks manually against the TasksContainer. This works locally
with Gradle 4.7+ for all scenarios in the sample (samples/local-spec).
There may be edge cases that I'm unaware of, and we may want to consider
defining the minimum supported Gradle version of 4.10 in the next major
version of openapi-generator-gradle-plugin if we experience those cases.

* Uncomment snapshots repo (commented it during local testing)

* update pom.xml for exec gradle plugin

modules/openapi-generator-gradle-plugin/samples/local-spec/README.md

@@ -17,5 +17,5 @@ gradle generateGoWithInvalidSpec
 The samples can be tested against other versions of the plugin using the `openApiGeneratorVersion` property. For example:
 
 ```bash
-gradle -PopenApiGeneratorVersion=3.2.3 openApiValidate
+gradle -PopenApiGeneratorVersion=3.3.0 openApiValidate
 ```