Use FoundationEssentials when available

[lemonmojo]

This PR replaces all occurrences of

import Foundation

with

#if canImport(FoundationEssentials)
import FoundationEssentials
#else
import Foundation
#endif

The benefit is that for platforms that do support FoundationEssentials (ie. non-Apple platforms), it results in less Swift library dependencies and ultimately in a smaller file size of the final distributable package.

There are two additional changes in this PR that could be removed if so desired:

1. Adds workflow_dispatch to the CI yml files to allow manually triggered runs. (This was useful for testing in the forked repo)
2. Fixes an issue (probably with newer Swift/Xcode versions) that lead to a compilation issue because the reserved keyword unsafe was used in RSASecKeyTests.swift.

All tests completed successfully locally on macOS using either Xcode or swift test and on Github's CI machines.

Update 1:
So to check if libFoundation.so is actually not(!) linked in on Linux I switched the output type of the target to .dynamic, did a swift build and objdump -p libCryptoSwift.so. Unfortunately it was still there. So I investigated and found out that this is caused by copying the PrivacyInfo.xcprivacy file. It very much sounds like a Swift/SPM bug to me. As soon as I remove the resource, only libFoundationEssentials.so is linked, not libFoundation.so. So I made another change, this time to Package.swift to exclude that file when compiling for non-Apple platforms. Unfortunately, the change is pretty hacky as it will not work when cross-compiling from macOS to Linux for instance.
If anyone knows why this is happening or has a better idea on how to handle this, I'm all ears.

Update 2:
Refactored the resources to be contained in a separate target which is added as a dependency of CryptoSwift only for Apple platforms. This gets rid of the cross-compilation issue from Update 1.