ENH: Shodan parser: handle invalid hostnames, move _common_keys out of class, adjust apply_mapping

[monoidic]

This PR makes some changes to the Shodan parser, namely:

• Instead of keeping track of extra.ftp.<something>.parameters, FTP parameters are collected together into extra.ftp.features as a list of said features, reducing field count
• rsync.modules is collected
• conversion functions can raise NoValueException with a string argument to signify that the conversion would not succeed, such as in the case of a single IP address being given in hostnames, which would then be passed into source.reverse_dns and fail to validate as a FQDN
• _common_keys is moved out of the class
• _dict_dict_to_obj_list is introduced, for converting a string-to-dict mapping into a list of dicts with the previous key as an attribute of the dict; this can be useful for preventing issues where, when feeding the data into aggregating tools, you'd end up with many more fields than necessary, e.g vulns.CVE-2010-0001.cvss, CVE-2010-0002.cvss etc.
• _get_first to get the first item from a list, with NoValueException raised on empty lists
• _get_first_hostname to handle the first valid FQDN from a list of hostnames for hostnames in the Shodan banner, if there is one, and gives NoValueException otherwise
• ssl.cert.serial and ssl.dhparams.generator, which may return both integers and strings, are converted to strings
• Changes to apply_mapping, such as reducing needless loop iterations, removing a big try-except, and adding the NoValueException handling described above
• Stops falsy values (False, 0) besides None from being filtered out

[codecov-commenter]

Codecov Report

Merging #2117 (c812b4b) into develop (2da82cf) will increase coverage by 0.01%.
The diff coverage is 86.66%.

@@             Coverage Diff             @@
##           develop    #2117      +/-   ##
===========================================
+ Coverage    75.90%   75.92%   +0.01%     
===========================================
  Files          440      440              
  Lines        23573    23605      +32     
  Branches      3150     3151       +1     
===========================================
+ Hits         17894    17922      +28     
- Misses        4947     4952       +5     
+ Partials       732      731       -1     

Impacted Files	Coverage Δ
intelmq/bots/parsers/shodan/parser.py	90.09% <86.20%> (-1.46%)	⬇���
intelmq/tests/bots/parsers/shodan/test_parser.py	100.00% <100.00%> (ø)

[waldbauer-certat]

Looks good for me

[ghost]

Thanks!

I add a changelog entry in 58b8423, heavily based on your PR description above.