Adding --dry-run option for merge.

[louib]

Type of change

• ✅ Refactor
• ✅ New feature
• ✅ Documentation

Description and Context

Two changes in fact: I added the --dry-run option and the list of changes detected by the merge operation to the output. In the future, I'd also like to ask the user for confirmation before merging, after displaying the changes.

Screenshots

$ keepassxc-cli merge -s --dry-run ~/test1.kdbx ~/test2.kdbx 
Insert password to unlock /home/louib/test1.kdbx: 
	Creating missing entry2 [06009d6280a741f88c4da6e4415d0aa5]
Database was not modified by merge operation.
$ keepassxc-cli merge -s ~/test1.kdbx ~/test2.kdbx 
Insert password to unlock /home/louib/test1.kdbx: 
	Creating missing entry2 [06009d6280a741f88c4da6e4415d0aa5]
Successfully merged the database files.
$ keepassxc-cli merge -s ~/test1.kdbx ~/test2.kdbx 
Insert password to unlock /home/louib/test1.kdbx: 
Database was not modified by merge operation.
$ keepassxc-cli merge -s --dry-run ~/test1.kdbx ~/test2.kdbx 
Insert password to unlock /home/louib/test1.kdbx: 
Database was not modified by merge operation.

Testing strategy

unit test + CLI (see screenshot)

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
• ✅ I have compiled and verified my code with -DWITH_ASAN=ON. [REQUIRED]
• ✅ My change requires a change to the documentation, and I have updated it accordingly.
• ✅ I have added tests to cover my changes.

[louib]

Closing in favor of #3254