Merge | Kernel32 Interop Namespace

[benrr101]

Description: This PR is one more part in the story of wrangling the interop code into an interop namespace. This time, the focus is on the Kernel32.dll interop, associated structures.

Important things in this PR:

• Moving Interop.Errors to Interop namespace, renaming to SystemErrors
• Moving Interop.UNICODE_STRING to Interop namespace, renaming to UnicodeString
• Extracting kernel32 structs/enums from files with >1 struct/enum
• Combining dllimports into a single Kernel32 class

Testing: This is just moving classes around, no functional changes made. Projects still build, so we should be good.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Project coverage is 72.30%. Comparing base (69ac9c5) to head (de5efe9). Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/Microsoft/Data/SqlTypes/SqlFileStream.Windows.cs	0.00%	11 Missing :warning:
...rc/Microsoft/Data/SqlClient/SNI/LocalDB.Windows.cs	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2978 +/- ## =========================================== - Coverage 92.58% 72.30% -20.28% =========================================== Files 6 288 +282 Lines 310 59660 +59350 =========================================== + Hits 287 43137 +42850 - Misses 23 16523 +16500 ``` | [Flag](https://app.codecov.io/gh/dotnet/SqlClient/pull/2978/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | Coverage Δ | | |---|---|---| | [addons](https://app.codecov.io/gh/dotnet/SqlClient/pull/2978/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `92.58% <ø> (ø)` | | | [netcore](https://app.codecov.io/gh/dotnet/SqlClient/pull/2978/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.40% <0.00%> (?)` | | | [netfx](https://app.codecov.io/gh/dotnet/SqlClient/pull/2978/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `70.68% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet#carryforward-flags-in-the-pull-request-comment) to find out more.

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