feat: add endpoints for networks

[jungwire]

Negotiator pull request:

Description:

Adds REST API endpoints for the networks. Exposes:

• GET /networks
• GET /networks/{id}
• GET /networks/{id}/negotiations
• GET /networks/{id}/managers
• GET /networks/{id}/resources

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
• [ ] I have updated the documentation in all relevant places

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.18576% with 64 lines in your changes missing coverage. Please review.

Project coverage is 79.93%. Comparing base (758e582) to head (1dc75fe).

Files	Patch %	Lines
...eric/negotiator/mappers/NetworkModelAssembler.java	68.18%	20 Missing and 8 partials :warning:
.../negotiator/mappers/NegotiationModelAssembler.java	60.00%	14 Missing and 4 partials :warning:
...ric/negotiator/mappers/ResourceModelAssembler.java	62.50%	12 Missing and 3 partials :warning:
..._eric/negotiator/dto/network/NetworkCreateDTO.java	88.88%	1 Missing :warning:
.../bbmri_eric/negotiator/dto/network/NetworkDTO.java	90.00%	1 Missing :warning:
...c/negotiator/mappers/NetworkCreateModelMapper.java	92.30%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #324 +/- ## ============================================ - Coverage 80.02% 79.93% -0.10% - Complexity 1123 1199 +76 ============================================ Files 151 157 +6 Lines 3350 3673 +323 Branches 185 197 +12 ============================================ + Hits 2681 2936 +255 - Misses 546 598 +52 - Partials 123 139 +16 ``` | [Flag](https://app.codecov.io/gh/BBMRI-ERIC/negotiator/pull/324/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/324/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BBMRI-ERIC) | `79.93% <80.18%> (-0.10%)` | :arrow_down: | 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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