introduce customBootRun property for customizing runtime parameter for Spring application

[litols]

Motivation:

resolve #75

Modification:

introduce customBootRun. This is propety-customizer for bootRun which is used in this plugin.
the users can specify custom parameter like following:

openApi {
    customBootRun {
        args = ["--spring.profiles.active=different-url"] 
    }
}

If the parameter is not specified by user, this plugin uses the parameter specified in bootRun in Spring Boot Gradle Plugin instead.
As a result, we can satisfy the following requirement this plugin users.

• the case that the users need specifing static properties to generate openapi schema(by the way, this includes me!)
• the case that the users need to try this plugin quickly

Notes:

• springdoc-openapi-gradle-plugin 1.3.4 had removed forkProperties. it is breaking changes for the users... also the user needs to migrate to customBootRun to upgrade this plugin continuously, so I recommend that you might want to release this changes as version 1.4.0.

[litols]

@bnasslahsen sorry for mention by name directly, please take a look this PR if you have a time?

[thattolleyguy]

Would love to see this merged and released

[juliaisnotavailable]

Would also love to see this released. The previous release 1.3.4 introduced a breaking change with regards to the forkProperties config. This would be a very welcome update.

[bnasslahsen]

Thank you @litols for your contribution to the project!