Enhance YAML API to configure supported features for section/subsection types

[dakochik]

• [ ] Add the ability to configure new section (e.g. via YAML file & UI in plugin settings) and specify section features:
  • is section value accessible after rules.rule_name.<>
• [ ] Also, support code completion (keywords and rules.rule_name.<> if applicable)

Fixed in ~~ (#536):

• ~~section name~~ (#536)
• ~~are lambdas allowed as section arguments (e.g. container doesn't allow)~~ (#536)
• ~~are wildcards allows as section arguments (e.g. container doesn't allow)~~ (#536)
• ~~does section accept keyword args (e.g. conda not allows, input allows)~~ (#536)
• ~~does the section allows only a single argument (e.g. like 'conda')~~ (#536)
• ~~is section is execution section (e.g. like 'shell')~~ (#536)
• ~~is section worflow/rule/checkpoint section~~ (#536)
• ~~snakemake version where the section is introduced (could be empty)~~ (#536)

• ~~[x] Show currently supported section in the above UI dialog + move them to YAML config~~ (#536)

Related: #334