Use FoundationEssentials

[adam-fowler]

These changes are all the easy ones where it is basically a swap out of the original import Foundation, plus dropping the iso8601 dateformatter

This is taking on the task @t089 started. This PR will include just the easy conversions first. I'll do others in separate PRs.

[codecov]

Codecov Report

❌ Patch coverage is 64.86486% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.27%. Comparing base (510e4ca) to head (c80b4ac).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/SotoXML/XMLDecoder.swift	55.55%	8 Missing ⚠️
Sources/SotoXML/XMLEncoder.swift	57.14%	3 Missing ⚠️
...s/SotoCore/Credential/Login/TokenFileManager.swift	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #683      +/-   ##
==========================================
- Coverage   80.48%   80.27%   -0.21%     
==========================================
  Files          88       88              
  Lines        6158     6170      +12     
==========================================
- Hits         4956     4953       -3     
- Misses       1202     1217      +15     

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[adam-fowler]

Oh this is going to be fun. Check out the dance that is needed to able to use FormatStyles from FoundationEssentials

soto-core/Sources/SotoXML/XMLEncoder.swift

Lines 541 to 551 in 58b5dc0

	#if canImport(FoundationEssentials)
	return XML.Element(name: self.currentKey, stringValue: date.formatted(Date.ISO8601FormatStyle(includingFractionalSeconds: true)))
	#else
	if #available(iOS 15, macOS 12, tvOS 15, watchOS 8, *) {
	return XML.Element(name: self.currentKey, stringValue: date.formatted(Date.ISO8601FormatStyle(includingFractionalSeconds: true)))
	} else {
	let formatterWithSeconds = ISO8601DateFormatter()
	formatterWithSeconds.formatOptions = [.withFullDate, .withFullTime, .withFractionalSeconds]
	return XML.Element(name: self.currentKey, stringValue: formatterWithSeconds.string(from: date))
	}
	#endif