refactor: Refactoring DataSource to DiscoveryService and updating tests

[jungwire]

Negotiator pull request:

Description:

Rename and simplify datasource table to discoveryservice

Checklist:

Make sure you tick all the boxes bellow if they are true or do not apply:

• [x] I have performed a self review of my code
• [x] My code follows Google Java Code style
• [x] I have made my code as simple as possible
• [x] I have added unit tests and the code coverage has not decreased
• [x] I have updated the documentation in all relevant places

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.96721% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 78.54%. Comparing base (9d0c60b) to head (666f510).

Files	Patch %	Lines
...ic/negotiator/database/model/DiscoveryService.java	53.33%	4 Missing and 3 partials :warning:
...egotiator/service/DiscoveryServiceServiceImpl.java	91.66%	2 Missing :warning:
.../bbmri_eric/negotiator/database/model/Request.java	0.00%	1 Missing :warning:
...bbmri_eric/negotiator/database/model/Resource.java	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #199 +/- ## ============================================ + Coverage 77.76% 78.54% +0.77% + Complexity 909 895 -14 ============================================ Files 122 122 Lines 2779 2740 -39 Branches 165 154 -11 ============================================ - Hits 2161 2152 -9 + Misses 509 490 -19 + Partials 109 98 -11 ``` | [Flag](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/199/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/199/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | `78.54% <81.96%> (+0.77%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC#carryforward-flags-in-the-pull-request-comment) to find out more.

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