Feat/add md ds to connectors

[Cervangirard]

Add the possibility to store the datasources object inside a connectors :
Main changes are :

• all functions to recreate datasources
• datasource is an attribute of connectors obj
• new class for nested connectors : nested_connectors
• new logic to use the datasources from connect or recreate it if the user doesn't use connect

How to test :

• use connect with a custom yaml file and use the datasources fct
• use the connectors fct with custom connector fct and unse the datasources fct

[vladimirobucina]

Functionalities look ok. I just think we need to stay consistent when definin imports from packages in documentation. I had few other comments. Also don't know if we should enforce parameter checking in internal functions (at least checkmate). All in all it looks good.

[github-actions]

Code coverage

Name	Coverage (%)
connector	76.23
R/cnt_generics.R	35.71
R/connect_utils.R	100.00
R/connect.R	95.43
R/connector.R	14.29
R/connectors.R	64.15
R/conts_datasources.R	97.85
R/dbi_backend_tools.R	66.67
R/dbi_methods.R	100.00
R/dbi.R	100.00
R/fs_backend_tools.R	66.67
R/fs_methods.R	100.00
R/fs_read.R	42.86
R/fs_write.R	66.67
R/fs.R	100.00
R/generic_backend.R	96.55
R/utils_fileext.R	100.00
R/utils_files.R	100.00
R/utils-docs.R	0.00
R/utils-roxygen.R	0.00

[vladimirobucina]

Now's good.