The code was already correct, but Spotbugs did not know that the methods
called were pure, and therefore thoought they might return null when
called a second time. This change enables Spotbugs to reason correctly
about the code.

Earlier, the settings page on Jenkins would crash if there were no other
plugins that pulled in okhttp3.

See https://www.jenkins.io/doc/developer/handling-requests/deprecated-reflective-access/

Makes changes to add new types of credentials less invasive, thus easier
to review.

This change lets users pick `StringCredentials` in addition to
`UsernamePasswordCredentials` in the credentials drop-down in the
settings. This makes the `SigningInterceptor` send the secret of the
credential as a bearer token, instead of using username and password for
Basic Authentication. See
https://confluence.atlassian.com/enterprise/using-personal-access-tokens-1026032365.html
for the documentation on the Jira side.