Allow the inputSpec in the Docs goal to be a directory instead of a single file

[torrespro]

If we could point to a folder we could generate the docs of multiple specs without the need for multiple executions.

Lint already supports dir:

backbase-openapi-tools/boat-maven-plugin/src/main/java/com/backbase/oss/boat/AbstractLintMojo.java

Line 26 in 5b750d8

@Parameter(name = "inputSpec", property = "inputSpec", required = true)

backbase-openapi-tools/boat-maven-plugin/src/main/java/com/backbase/oss/boat/AbstractLintMojo.java

Line 54 in 5b750d8

if (inputSpec.isDirectory()) {