Add various cleanliness changes

[punmechanic]

• Replace custom hand-rolled HTTP calls with the oidc/oauth2 packages
• Move oauth2 handling code to oauth2/ package.
• Use newer Go functions like slices.Contains instead of our hand-rolled functions
• Simplify folder structure by moving commands outside of main package, and move main package to root folder so go run . works.
• Migrate to Lambda function extension for Vault secret handling, so we don't need to handle it in the application.
• Remove usage of the custom Riot Vault SDK, using the Hashicorp one instead.
• Upgrade to v2 of the AWS SDK.
• Pass HTTP client through context and remove go-rootcerts as it's no longer needed in newer Go versions.
• Put binaries in bin/ instead of a custom directory.
• Disable CGO.
• Hide OIDC flags that no one actually uses.
• Remove partial Tencent Cloud support. We will add this back, but we don't want regular usage of the application to panic.
• Refactor environment variable writing so we rely less on string interpolation.

Resolves #111; using the OAuth2 library provides an OAuth2-related error when exchanging an access and id token for a web sso token, whereas previously this was silently ignored and would lead to a cryptic error when retrieving a SAML assertion.