Allow to obtain field name property from field context.

[rthideaway]

This would be a useful info to get as it is kind of context which could be used in data producers. I can illustrate it on one of our data producers.

We have data producer doing search api solr query:

// Search API jobs query.
$registry->addFieldResolver('Query', 'jobs', $builder->compose(
  $builder->context('search_id', $builder->fromValue('jobs_search')),
  $builder->produce('search_api_load', [
    'index_id' => $builder->fromValue('job_search'),
    'language' => $builder->fromArgument('language'),
    'range' => $builder->fromArgument('ranges'),
    'sort' => $builder->fromValue([]),
    'fulltext' => $builder->fromArgument('fulltext'),
  ])
));

After execuion of search api query we do some alters to solarium query to use some specific solr features which cannot be used within SearchApiQuery object. We do that in hook_search_api_solr_query_alter. Our data producer sets search id so it's easily target-able in that hook:

// Set search id context to the query so it's easily target-able.
if ($search_id = $context->getContextValue('search_id')) {
  $this->query->setSearchId($search_id);
}

This would be much easier if we could obtain fieldName property of resolve info within FieldContext because it's unique. So instead of creating some custom context we'd just use this:

    $query->setSearchId($fieldContext->getFieldName());

This PR should make it possible.

[codecov]

Codecov Report

Merging #929 into 8.x-4.x will decrease coverage by 0.08%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             8.x-4.x     #929      +/-   ##
=============================================
- Coverage      61.63%   61.54%   -0.09%     
- Complexity       678      679       +1     
=============================================
  Files             95       95              
  Lines           1405     1407       +2     
=============================================
  Hits             866      866              
- Misses           539      541       +2

Impacted Files	Coverage Δ	Complexity Δ
src/GraphQL/Execution/FieldContext.php	22.22% <0%> (-2.78%)	7 <1> (+1)

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