added new backend - postgres db

[grittyashish]

A new backend for fetching data from postgres db.
This backend works with SSL and non-SSL configured postgres databases.
For SSL postgres db, provide location of certificates and client key and choose the appropriate ssl mode.
This backend can be easily modified to add new categories, aka queries.

Provide the URL/endpoint, database name, username, password file's location and category as environment variable
usage : perceval postgresdb --url $url --db $db --usr $usr --passw $passw --category $CATEGORY

Tested using local postgres database running on port 5432(default postgres port).

Attached are outputs from the test run :

[grittyashish]

@jjmerchante @sduenas Unable to add any of you as reviewer so tagging here.
Please let me know your views on this and any modifications that you require in this in order for it to merge into main.

[GeorgLink]

Hi @grittyashish , thank you for opening the PR.

I want to understand the use case and your motivation for creating this. Was there an issue or some other discussion for the need for this new feature?