chore: Add spectral lint rule

[blva]

Proposed changes

Add spectral lint to validate custom methods

Example:

INFO: Build completed successfully, 1 total action
Running spectral linter
6.11.1
 - linting /Users/bianca.lisle/workplace/mms/server/openapi/services/openapi-mms.yaml

/<redacted>/openapi-mms.yaml
 1640:69  error  no-slash-before-custom-method  Ensure that paths do not contain a '/' immediately before a custom method (e.g., ':applyItem').  paths./api/atlas/v2/orgs/{orgId}/invoices/{invoiceId}/lineItems/:search

Checklist

• [x] I have signed the MongoDB CLA
• [x] I have added tests that prove my fix is effective or that my feature works

Further comments