feat: loader for extensions in custom base packages

[pablocarle]

Description

Linked to #1986

Implement a library for loading extensions which are defined in their own package name, it's provided as a library implementation to allow reusing in different services (for example Discovery Service and Gateway Service can use it to load these)

Based on a change to the manifest files of the extensions where if the extension defines a different base package it is scanned.
The scan is done by hooking into ApplicationContextInitializedEvent

Type of change

Please delete options that are not relevant.

• (feat) New feature (non-breaking change which adds functionality)
• (docs) Change in a documentation

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

For more details about how should the code look like read the Contributing guideline

[pablocarle]

PR in docs-site: zowe/docs-site#2042

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

78.1% Coverage
0.0% Duplication

[balhar-jakub]

The coverage is incorrectly calculated from files we didn't touch here.