feat: Add API endpoint to get available slots for a room

[waveyboym]

Description

This pr adds an api endpoint that will allow the mobile to request all available slots for a specific room on a specific date

Fixes # (issue)
#331

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Wrote unit tests to test ability of open slots compute function
• Wrote unit tests to test database function under various test cases

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.68%. Comparing base (94f5a4d) to head (3e08b9e).
Report is 7 commits behind head on develop.

Files	Patch %	Lines
occupi-backend/pkg/database/database.go	88.23%	1 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (94f5a4d) and HEAD (3e08b9e). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (94f5a4d)	HEAD (3e08b9e)
	3	1

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #332      +/-   ##
===========================================
- Coverage    79.04%   73.68%   -5.36%     
===========================================
  Files           53       10      -43     
  Lines         3164     1923    -1241     
  Branches        81        0      -81     
===========================================
- Hits          2501     1417    -1084     
+ Misses         576      459     -117     
+ Partials        87       47      -40     

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