fix(rdb): update endpoints handling

[Mia-Cross]

This PR changes the way endpoints are handled to integrate the latest changes in the RDB API: if a private network is defined, there is no more default public endpoint. So if the user wants both a private and a public endpoint, they must explicitly declare a load_balancer {} empty block alongside the private_network. This also means there is no need for the disable_public_endpoint field anymore.
Closes #2281

Also there was a problem with the updates of private network endpoints, that did not always use IPAM as they should.
Closes #2299

The PR should also fix the nightly tests.

[Mia-Cross]

Examples :

scaleway_rdb_instance example {
    private_network {
        pn_id = xxx
        (enable_ipam = true)
    }
}

--> Defines an IPAM configured private network endpoint

scaleway_rdb_instance example {
    private_network {
        pn_id = xxx
        enable_ipam = false
    }
    load_balancer {}
}

--> Defines a static private network endpoint and a load_balancer public endpoint

scaleway_rdb_instance example {
}

--> Defines a default load_balancer public endpoint

[codecov-commenter]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (0ccefa7) 73.64% compared to head (ccab632) 73.62%.

Files	Patch %	Lines
scaleway/resource_rdb_instance.go	68.08%	11 Missing and 4 partials ⚠️
scaleway/helpers_rdb.go	74.19%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2308      +/-   ##
==========================================
- Coverage   73.64%   73.62%   -0.02%     
==========================================
  Files         218      218              
  Lines       33848    33866      +18     
==========================================
+ Hits        24928    24935       +7     
- Misses       6787     6797      +10     
- Partials     2133     2134       +1     

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