[JAXRS-spec] Make path for openapi.yaml configurable

[michbeck100]

Currently the path of the generated openapi.yaml is always "src/main/openapi" (see here). This should be configurable, because the microprofile openapi spec, for example, needs the openapi.yaml at the META-INF folder.

[jmini]

I totally agree with this request because this allow to support MicroProfile without annotation (see discussion in #795).

I have also seen your PR #813 that is also a step to support "MicroProfile without annotation". I will review it ASAP.

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In my opinion we should have a new generator for that:
java-microprofile-without-annotation that configures the options of JAXRS-spec and also create the correct README and pom.xml to be able to bootstrap a MicroProfile in a user-friendly way.

It will be a reuse of almost everything of "JAXRS-spec".

What do you think?