PostgreSQL: initial support

[jufajardini]

• pgsql/parser: add startup message structure, initial nom parser and corresponding unit tests
• pgsql/parser: add a parser for the the 'user' startup parameter and corresponding unit tests
• lib: add pgsql to the module's list

Make sure these boxes are signed before submitting your Pull Request -- thank you.

• I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
• I have signed the Open Information Security Foundation contribution agreement at https://suricata-ids.org/about/contribution-agreement/
• I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket:

• https://redmine.openinfosecfoundation.org/issues/4241

Link to previous PR: #5706

Describe changes:

• pgsql/parser: add a parser for the the 'user' startup parameter and corresponding unit tests
• pgsql/parser: add constants for dummy protocol versions used for some message types; and for data type codes
• StartupMessage parser: add unit test for invalid length
• nit: replace 2020 with 2021 in the Copyright statement

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[jufajardini]

Closed with: #5747