feat: allow secrets to be passed via environment variables

[jeremylong]

Any secrets should be configurable/retrievable via environment variables as opposed to arguments or mave/gradle/ant configuration.

[marcelstoer]

Addendum

• This is currently only available for the NVD API key as nvdApiKeyEnvironmentVariable.
• As per the discussion at feat: Allow configuring OSS Index user/pw directly #7640 (comment) maybe we don't need to make this available for all secrets but limit it to the ones for external resources (NVD, OSS, etc.).

[ApprenticeofEnder]

Addendum

* This is currently only available for [the NVD API key](https://github.com/dependency-check/DependencyCheck/blob/main/maven/src/main/java/org/owasp/dependencycheck/maven/BaseDependencyCheckMojo.java#L2481-L2499) as `nvdApiKeyEnvironmentVariable`.

* As per the discussion at [feat: Allow configuring OSS Index user/pw directly #7640 (comment)](https://github.com/dependency-check/DependencyCheck/pull/7640#issuecomment-2879531580) maybe we don't need to make this available for all secrets but limit it to the ones for external resources (NVD, OSS, etc.).

Would it not make more sense to have it be standardized as something like NVD_API_KEY=... dependency-check <args>? This way you're not relying on setting a flag to accept an environment variable, which can be a bit of a headache in terms of UX.