DnsRecordMx: Implemented as a class-based resource

[Sudman1]

Pull Request (PR) description

Implements a class-based solution for managing MX records - to include implementation of managing scoped records.

Note: This PR also removes xDnsRecordMX to avoid conflicts.

This Pull Request (PR) fixes the following issues

• Fixes #140

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [x] Resource documentation added/updated in README.md.
• [x] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help. Does not use schema.mof file. Parameter Descriptions not updated in the README.md, as that has moved to the Wiki.
• [x] Comment-based help added/updated.
• [x] Localization strings added/updated in all localization files as appropriate.
• [x] Examples appropriately added/updated.
• [x] Unit tests added/updated. See DSC Community Testing Guidelines.
• [x] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [x] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #227 (6eee939) into main (01585ce) will decrease coverage by 4%. The diff coverage is n/a.

@@         Coverage Diff         @@
##           main   #227   +/-   ##
===================================
- Coverage    90%    85%   -5%     
===================================
  Files        16     16           
  Lines      1460   1537   +77     
===================================
- Hits       1319   1318    -1     
- Misses      141    219   +78     

Impacted Files	Coverage Δ
...Resources/MSFT_xDnsRecordMx/MSFT_xDnsRecordMx.psm1	0% <0%> (-99%)	:arrow_down:
source/xDnsServer.psm1	99% <0%> (+<1%)	:arrow_up:

[johlju]

@Sudman1 I saw you reused the examples, integration tests and unit tests, let's leave them as is. Just revert the code folder xDnsRecordMx and add a deprecated message in the change log.

[johlju]

Tracking the removal of the old resource here https://github.com/dsccommunity/xDnsServer/issues/228.

[johlju]

@Sudman1 seems it fails on the xDnsRecordMx unit test. You can just delete that unit test. There are no changes introduced in this PR that changes that resource, right?

[johlju]

...also the integration test for xDnsRecordMx can be removed... it also fails.

[Sudman1]

That's correct. Thanks - you've saved me some Pester Mock troubleshooting there.