Encrypt http basic credentials

[MarcialRosales]

Proposed Changes

Implement feature requests:

• https://vmw-jira.broadcom.net/browse/RMQ-3752 (encrypted http basic credentials)
• https://vmw-jira.broadcom.net/browse/RMQ-2544 (move all cookie management to server-side so that we can fully secure them)

docs pr: rabbitmq/rabbitmq-website#2543

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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[michaelklishin]

This PR has the same regression we have fixed recently:
for OAuth 2-enabled deployments, oauth_completeLogin calls set_token_auth in a way that seemingly hardcodes the 8 hour timeout.

selenium/test/basic-auth/rabbitmq.encrypted-credentials.conf:1
still has management.credentials_encryption_secret ("credentials" in plural) vs. the rename singular Cuttlefish schema key.
This part I will correct myself since it's clear what should be done.