Fix the cspell job in GitLab CI pipeline

Problem/Motivation

GitLab CI fails the cspell job in the standard pipeline because we have a bunch of unrecognized words. For a recent example, see job #8672061 from just before the 4.4.0 release.

Proposed resolution

• Add _CSPELL_WORDS: 'idps, nameid, onelogin, samlp, seckit, urlencoding' to .gitlab-ci.yml to cover those words across multiple files
• Add <!-- cspell:disable --> ... <!-- cspell:enable --> around the list of maintainer names in README.md
• Change $rvaa and $arra to $r_v_a_a and $a_r_r_a in saml_sp_drupal_login.module
• Update the variable names we control and add // cspell:ignore whitelist to SamlSPDrupalLoginController.php and SamlSpDrupalLoginSubscriber.php while we wait for #3541096: Remove the term whitelist* from the module in the SecKit module
• Add // cspell:ignore authnsign, spsso, wsign to SamlSPMetadata.php
• Fix various singletons in a few files

Remaining tasks

Consider whether to restore the issue originally identified by @benstallings in #3546849: Standardize English spelling: lowercaseUrlencoding should likely be lowercaseUrlEncoding in config/install/saml_sp.settings.yml, config/schema/saml_sp.schema.yml, saml_sp.module, and src/Form/SamlSpConfig.php.

This will require an update hook and anyone who has saved their config to re-export, so it’s probably not worthwhile until we’re also updating something else in the settings.

But also, cspell allows Urlencode and Urlencoded, so we could argue that we don’t need to change it at all.