Allow customizable authentication methods in SFTP client

[Nuvindu]

Description:

Currently, the Ballerina SFTP connector supports several authentication methods such as gssapi-with-mic, publickey, keyboard-interactive, and password. However, user can't specify only the auth methods they require at the moment,
which may lead to some usability issues such as getting unnecessary Kerberos auth requirements each time a SFTP client is initialized (This can be fixed by only using publickey instead of having keyboard-interactive)

[github-actions]

This issue is NOT closed with a proper Reason/ label. Make sure to add proper reason label before closing. Please add or leave a comment with the proper reason label now.

      - Reason/EngineeringMistake - The issue occurred due to a mistake made in the past.
      - Reason/Regression - The issue has introduced a regression.
      - Reason/MultipleComponentInteraction - Issue occured due to interactions in multiple components.
      - Reason/Complex - Issue occurred due to complex scenario.
      - Reason/Invalid - Issue is invalid.
      - Reason/Other - None of the above cases.

[dilanSachi]

Related - #6764