Align flag naming according to the discussion

[ReToCode]

Context

According to the discussion in https://docs.google.com/document/d/1b-aCADlUBo2FZEDp5z5XoXlfUT6CvED339fcAdKY66o and https://docs.google.com/document/d/1rpag5-zffHGxAT7V4Nv28C_xx5Ow6L4mZuHbe3ebOQ8, we agreed that we want to align the flags related to Serving Encryption.

Desired target state

We would like to have three flags to configure everything related to Serving Encryption:

• external-domain-tls: true, false (bool)
• cluster-local-domain-tls: "disabled", "enabled" (enum) - with options to add mutual, identity later
• system-internal-tls: "disabled", "enabled" (enum) - with options to add mutual, identity later

Please note, the existing flags need to still work with the following mapping:

• autoTLS = external-domain-tls
• system-internal-tls= internal-encryption

Work

• Add the flags in networking, make sure that the old flags are mapped to the new ones
• Update all code usages, e.g. networking, pkg (if any), serving, all net-* repos -> Use the aligned flags and certificate secrets in all components #14369 (comment)
• Update the documentation, make sure to only add documentation where the feature is currently available