[FEAT] [no-unnecessary-type-assertion] Add rule

[uniqueiniquity]

• I’ve filled out the documentation, including all relevant sections and a link to the equivalent TSLint rule if available.
• I’ve filled out the rule’s category
• I’ve added documentation for the equivalent TSLint rule (in extraDescription)
• I’ve added the rule documentation link via the helper from ../utils.
• I’ve added the rule to the roadmap

• I’ve added tests for my changes

Adding equivalent of TSLint's no-unnecessary-type-assertion. Depends on eslint/typescript-eslint-parser#568 for type information.

[uniqueiniquity]

@j-f1 you may have opinions here and on #250 about adding TS as a peer dependency. I can match whatever makes sense for #265.

[armano2]

@uniqueiniquity do you mind adding helper function to utils?
with better message xd

 /**
  * Try to retrieve typescript parser service from context
  * @param {RuleContext} context Rule context
  * @returns {object} parserServices
  */
 exports.getParserServices = context => {
     if (
         !context.parserServices ||
         !context.parserServices.program ||
         !context.parserServices.esTreeNodeToTSNodeMap
     ) {
         throw new Error(
             "This rule requires you to use `typescript-eslint-parser`."
         );
     }
     return context.parserServices;
 };

[bradzacher]

@armano2 added as a separate PR #270, so that other PR's aren't dependent on this PR.