Various improvements

[stelabouras]

Improve CDSHandler push logic

The pushTranslations() method now returns a secondary array of Error
objects in the completion handler that represents the generated warnings
during the processing of the source strings that are non-fatal but can
be of interest to the developer.

This change introduces a new enum (TXCDSWarning) that hosts those
warnings, like duplicate source string and empty key detection.

The pushTranslations() logic has been improved by implementing the
following changes:

• serializeTranslations() is now a private static method as it does
  not need to look into the state of the CDSHandler instance.
• That method now returns a tuple that includes a Result<Data> object
  as the first member, revealing whether the serialization
  was successful, thus offering the Data object, or a failure, thus
  offering a Error object for further propagation and an array of
  TXCDSWarning objects for any generated warnings during the processing
  of the source strings.
• The pushTranslations() implementation has been split so that it's
  easier to be read. The new method that performs the asynchronous network
  operation is now separate (pushData()) and propagates the generated
  warnings to the pollJobStatus() method.
• Any error logging has been removed in favor of the list of Error
  objects in the completion handler. The logging is now a responsibility
  of the consumer.

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Introduce t() method

A new public static method has been introduced in the TXNative class,
allowing developers to programmatically trigger the translation
mechanism of Transifex SDK in cases where this is not supported.

For example, in SwiftUI, where the swizzling mechanism of Transifex SDK
does currently work, developers can replace views like this:

Text("test string")

with this:

Text(TXNative.t("test string"))

The method uses the default locale that Transifex SDK has been
configured with.

Note: If you want to change the current locale used by the
SwiftUI preview to test different locales, click on the 'Product' menu
item and then visit 'Scheme' > 'Edit Scheme...' > 'Run' > 'Options' >
'App Language' and pick the language you want to test.