Address unmatched return warnings from Dialyzer

[seancribbs]

If you use JokenJwks.DefaultStrategyTemplate in your own code and run dialyzer
to check with -Wunmatched_returns, three warnings appear on the line where
use was put. Adding location: :keep to the quote in
JokenJwks.DefaultStrategyTemplate reveals the following lines:

  lib/joken_jwks/default_strategy_template.ex:105: Expression produces a value of type
          atom() | ets:tid(), but this value is unmatched
  lib/joken_jwks/default_strategy_template.ex:183: Expression produces a value of type
          {'ok', pid()}, but this value is unmatched
  lib/joken_jwks/default_strategy_template.ex:216: Expression produces a value of type
          'ok' | {'ok', pid()}, but this value is unmatched

This commit resolves the above warnings as following (respectively):

• The call to :ets.new/2 uses the :named_table option, and so should always
  return the atom that was passed as the table name, namely __MODULE__.
• The call to start_fetch_signers/2 has the same return type as
  Task.start/1, which is {:ok, pid()}, so we match on {:ok, _}.
• The call to check_fetch could return :ok or {:ok, pid()} based on the
  branches of the case expression inside the function. Since there weren't any
  simple ways to resolve this, I chose to match with _.

[victorolinasc]

Thanks a lot @seancribbs !

[seancribbs]

Thank you for the merge! ❤️🧡💛💚💙💜

[victorolinasc]

I will run master on some services we use it here and if all goes well I'll release a patch version