Add version number to the seeder

Summary:
The seeder does not currently have a version number. This will make it
easier to direct users to upgrade.

Test Plan:

ninja
./bitcoin-seeder --version

Verify the output is as follows (or similar if version changes before this lands):

Bitcoin ABC Seeder v0.21.4-2436d3037
./bitcoin-seeder --help

Verify that the version output above appears before the rest of the help text.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5617