Template Fragment Installation: Clarify 3rd-party template fragment warnings

[chrisrueger]

Update user-facing prompts in AddCommands.java and NewWorkspaceWizard.java to improve clarity and safety. The text now adds an explicit warning that these fragments may contain build instructions (e.g., .bnd files) which are executed on the next build, asking users to proceed only if they fully trust the authors/content.

Example CLI

bnd add fragment "IndexedMavenRepository"

Your selection would install 1 template fragments from 3rd-party authors (including required dependency fragments): 
IndexedMavenRepository - Provide an OSGi repository derived from a subset of one or more maven repositories
  - Repo: https://github.com/mnlipp/de.mnl.osgi/tree/master/de.mnl.osgi.bnd.repository/workspace-template 

These fragments may contain build instructions (e.g., .bnd files) that are executed with the next build. Only continue if you fully trust the authors and the content.
Do you want to proceed? (yes/no): 

# Yes

Added 1 template fragment(s): 
IndexedMavenRepository - Provide an OSGi repository derived from a subset of one or more maven repositories
  - Repo: https://github.com/mnlipp/de.mnl.osgi/tree/master/de.mnl.osgi.bnd.repository/workspace-template 

# No

No template fragments installed with workspace, because user did not confirm the installation.

Example bndtools