Implement function to clear all ssh-agent identities

[AlexpFr]

Implement a function that replicates the behavior of the ssh-add -D command.

• Fixes #8346

Testing strategy

Tested manually and compare result to the real ssh-add -D command. Ensure that the Tools menu item is hidden when the SSH agent is disabled or when -DWITH_XC_SSHAGENT=OFF.

Type of change

• ✅ New feature (change that adds functionality)

[AlexpFr]

Hello, is this PR ready for a merge? Do I need to review certain points? Thanks.

[droidmonkey]

Will review when I get the chance

[AlexpFr]

Remember note:

Openssh-portable upstream code relative to ssh-add -D:

• V_9_7_P1/ssh-add.c#L223
• V_9_7_P1/authfd.c#L730