Use FilterHelper method to generate wire:key, add placeholder with docs

[lrljoe]

Two key changes:

• For all Filters (Except MultiSelect currently), use the FilterHelper method to generate the wire:key rather than doing it in the blade. This centralises for future updates
  MultiSelectFilter will follow in due course, as it requires some tidying & updates.

• Add placeholder config option to: DateRangeFilter, DateFilter, DateTimeFilter, NumberFilter and update docs to reflect the availability of the option

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests and did you add any new tests needed for your feature?
2. Did you update all templates (if applicable)?
3. Did you add the relevant documentation (if applicable)?
4. Did you test locally to make sure your feature works as intended?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

Merging #1493 (7bd7688) into v3-develop (0cc1dea) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             v3-develop    #1493   +/-   ##
=============================================
  Coverage         88.08%   88.08%           
  Complexity         1251     1251           
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  Files                88       88           
  Lines              3013     3013           
=============================================
  Hits               2654     2654           
  Misses              359      359