feat: introduce a dedicated namespace for OTPs and SPPs

[srieteja]

- What I did

• introduce a dedicated namespace for OTPs and SPPs
• Provide backwards compatibility for legacy OTP and SPP keys

- How I did it

• Introduced a class variable in OtpVerbHandler called that has a value of '__otp'; which will be the namespace for all OTPs and SPPs
• Allow backward compatibility: check for OTP and SPP without the otp namespace.

- How to verify it

• Unit tests added that validate that OTPs and SPPs that use legacy style keys are also considered valid
• functional/e2e tests will be once this PR is merged into https://github.com/atsign-foundation/at_server/pull/1955

- Description for the changelog

feat: introduce a dedicated namespace for OTPs and SPPs

[srieteja]

Thank you @gkc