[automation] Manually add some nodes to the seeds

Summary: This diff checks if a file named extra_nodes_<chain>.txt exists in the seed directory and append it to the node list extracted from the seeder.

Test Plan:
Create 2 empty files: contrib/seeds/seeds_main.txt and contrib/seeds/seeds_test.txt
Add some ip:port to the main and test extra nodes files, then:

./contrib/source-control-tools/patch-recipe/update-seeds.h

The ip:port are added to the nodes_<chain>.txt list (the script will likely fail due to missing connections, this is norma).

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11931