add option to enable agent forwarding

[NikitaVasiliev]

This pull request introduces the forward-ssh-agent option, allowing users to leverage SSH agent forwarding when executing commands on remote hosts. This facilitates secure authentication by forwarding connections from a local authentication agent, such as ssh-agent, to the remote host. It’s particularly useful in environments where direct SSH access needs to be secured and simplified without exposing private key material.

If it isn't needed or doesn't fit the project's direction, feel free to decline this PR.

[umputun]

Thank you. Here are a few minor corrections:

• The linter failed on several trivial warnings.
• The test code for runSSHAgent does not seem to wait for the completion of both goroutines it created. I believe that sock.Close() will cause them to exit, but it may leave the goroutine running after the test is completed. Perhaps using a simple waitgroup with a wait after the Close will solve this issue.

[NikitaVasiliev]

There was indeed a problem. The simple use of a WaitGroup led to a deadlock because some goroutines were stuck at connections. I had to close those connections as well.

[coveralls]

Pull Request Test Coverage Report for Build 8591076724

Details

• 9 of 34 (26.47%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 84.001%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/executor/connector.go	6	31	19.35%

Totals
Change from base Build 8565636282:	-0.6%
Covered Lines:	2704
Relevant Lines:	3219

---

💛 - Coveralls

[umputun]

A few problems:

• Test_sshAgentForwarding fails on my mac:

    main_test.go:448: 
        	Error Trace:	/Users/umputun/dev.umputun/spot/cmd/spot/main_test.go:391
        	            				/Users/umputun/dev.umputun/spot/cmd/spot/main_test.go:448
        	Error:      	Received unexpected error:
        	            	listen unix : bind: invalid argument
        	Test:       	Test_sshAgentForwarding

• The test doesn't check much, and because this is the only test related to the connector change, it would be nice to make sure that the output of the command actually matches the expectation.

[NikitaVasiliev]

• The first problem is due to relying on undefined behaviour when provide empty address for the unix network to Listen function in Linux. This should have been fixed new.
• Now the test checks that the forwarded ssh-agent has the key with an expected fingerprint.

[umputun]

LGTM, thx