[BE] Modify Execution intervals for STT checks

User story:
When I go to Settings/configuration for STT I need to be able to change default values to Check Execution intervals so that I as a pmm Admin have control over the generated load on my system

UI/UX:
Note: Need to create a separate ticket for UI lets treat this one as backend

Acceptance criteria

• User able to change values for 3 buckets/intervals: frequently (4h) - standard (24h) - rare (78h)
• The user is able to enter 0.01h as a value (will be useful for debugging)

Out of scope:

Suggested implementation:

• Add 3 new fields (for e.g. `STTStandardInterval`, `STTRareInterval`, `STTFrequentInterval`) under the saas config options of the settings model used by managed and make required changes to the schema
• Make changes to the server endpoint `ChangeSettings` to save these three buckets to DB
• change the checks service code to retrieve settings from DB instead of using hardcoded value (if https://jira.percona.com/browse/PMM-7335 has already been implemented otherwise ignore this point)

How to test:

• Call GetSettings endpoint

Details:
TBD