Handle escape sequences correctly in authentication config

What does this MR do?

A customer scan fails with an error similar to: ERR MAIN configuration validation failed error="failed to parse TOML configuration due to (16, 13): invalid escape sequence: \\H"

This is due to the \ character not being escaped. We should escape all \ characters prior to inserting into the browserker.toml configuration file.

This MR converts the password and username to string representation before writing it to a file.

What are the relevant issue numbers?

gitlab-org/gitlab#355114 (closed)

GitLab Docs MR

Closes #355114

TODO:

• Add changelog